feat(android): About card check-for-updates button (audit v2 #20, slice 2)

Closes audit v2 #20.

* ClientVersionApi — GET /api/client/version returning
  UpdateInfo(version, apkUrl, sizeBytes).
* UpdateRepository.getLatest() + isVersionNewer() free function
  ported from Flutter's component-wise integer compare (handles
  our date-style 2026.05.10.1 versions correctly, with a
  branch-name fallback for non-numeric builds like dev/main).
* AboutCardViewModel surfaces a four-state UpdateCheckResult
  (Idle/Latest/UpdateAvailable/Error).
* About card grows a "Check for updates" button + inline status
  line. "Install vX.Y.Z" wiring is #25 (post-v1).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/models/UpdateInfo.kt

@@ -0,0 +1,15 @@
+package com.fabledsword.minstrel.models
+
+/**
+ * Wire shape returned by `GET /api/client/version`. Mirrors
+ * `flutter_client/lib/update/update_info.dart UpdateInfo`.
+ *
+ * `version` is the server-bundled APK version (may have a leading
+ * "v" from the git tag); `apkUrl` is server-relative (e.g.
+ * `/api/client/apk`); `sizeBytes` is the download size.
+ */
+data class UpdateInfo(
+    val version: String,
+    val apkUrl: String,
+    val sizeBytes: Long,
+)

android/app/src/main/java/com/fabledsword/minstrel/models/wire/UpdateInfoWire.kt

@@ -0,0 +1,15 @@
+package com.fabledsword.minstrel.models.wire
+
+import kotlinx.serialization.SerialName
+import kotlinx.serialization.Serializable
+
+/**
+ * Wire shape for `GET /api/client/version`. Defaults match Flutter:
+ * apk_url falls back to `/api/client/apk` if the server omits it.
+ */
+@Serializable
+data class UpdateInfoWire(
+    val version: String = "",
+    @SerialName("apk_url") val apkUrl: String = "/api/client/apk",
+    @SerialName("size_bytes") val sizeBytes: Long = 0,
+)

android/app/src/main/java/com/fabledsword/minstrel/settings/ui/AboutCardViewModel.kt

@@ -0,0 +1,69 @@
+package com.fabledsword.minstrel.settings.ui
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import com.fabledsword.minstrel.BuildConfig
+import com.fabledsword.minstrel.models.UpdateInfo
+import com.fabledsword.minstrel.update.data.UpdateRepository
+import com.fabledsword.minstrel.update.data.isVersionNewer
+import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.update
+import kotlinx.coroutines.launch
+import javax.inject.Inject
+
+/**
+ * One of three terminal states the Check-for-updates button surfaces.
+ * `Idle` is the pre-check state; `Latest` means the installed build
+ * matches or exceeds the server's bundled APK; `UpdateAvailable`
+ * means the server has a newer build (rendered as "Install vX.Y.Z"
+ * in slice 2 of #25 — for now it just shows the version label).
+ */
+sealed interface UpdateCheckResult {
+    data object Idle : UpdateCheckResult
+    data object Latest : UpdateCheckResult
+    data class UpdateAvailable(val info: UpdateInfo) : UpdateCheckResult
+    data class Error(val message: String) : UpdateCheckResult
+}
+
+data class AboutUiState(
+    val installedVersion: String = BuildConfig.VERSION_NAME,
+    val isChecking: Boolean = false,
+    val result: UpdateCheckResult = UpdateCheckResult.Idle,
+)
+
+/**
+ * Backs the About card's "Check for updates" button. Calls
+ * [UpdateRepository.getLatest] on tap, compares versus the build's
+ * VERSION_NAME via [isVersionNewer], and reports the terminal state.
+ */
+@HiltViewModel
+class AboutCardViewModel @Inject constructor(
+    private val repository: UpdateRepository,
+) : ViewModel() {
+
+    private val internal = MutableStateFlow(AboutUiState())
+    val state: StateFlow<AboutUiState> = internal.asStateFlow()
+
+    fun checkForUpdates() {
+        if (internal.value.isChecking) return
+        viewModelScope.launch {
+            internal.update { it.copy(isChecking = true) }
+            val installed = internal.value.installedVersion
+            val result = runCatching { repository.getLatest() }
+                .map { latest ->
+                    if (isVersionNewer(latest.version, installed)) {
+                        UpdateCheckResult.UpdateAvailable(latest)
+                    } else {
+                        UpdateCheckResult.Latest
+                    }
+                }
+                .getOrElse { e ->
+                    UpdateCheckResult.Error(e.message ?: "Couldn't reach server")
+                }
+            internal.update { it.copy(isChecking = false, result = result) }
+        }
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/settings/ui/SettingsScreen.kt

@@ -235,11 +235,12 @@ private fun displayName(mode: ThemeMode): String = when (mode) {
 }
 
 @Composable
-private fun AboutCard() {
+private fun AboutCard(viewModel: AboutCardViewModel = hiltViewModel()) {
+    val state by viewModel.state.collectAsStateWithLifecycle()
     ElevatedCard(modifier = Modifier.fillMaxWidth()) {
         Column(
             modifier = Modifier.padding(16.dp),
-            verticalArrangement = Arrangement.spacedBy(4.dp),
+            verticalArrangement = Arrangement.spacedBy(8.dp),
         ) {
             Text(
                 text = "About",
@@ -247,14 +248,47 @@ private fun AboutCard() {
                 color = MaterialTheme.colorScheme.onSurface,
             )
             Text(
-                text = "Minstrel ${BuildConfig.VERSION_NAME} (${BuildConfig.VERSION_CODE})",
+                text = "Installed: Minstrel ${state.installedVersion} " +
+                    "(${BuildConfig.VERSION_CODE})",
                 style = MaterialTheme.typography.bodySmall,
                 color = MaterialTheme.colorScheme.onSurfaceVariant,
             )
+            UpdateCheckLine(result = state.result)
+            Button(
+                onClick = viewModel::checkForUpdates,
+                enabled = !state.isChecking,
+                modifier = Modifier.fillMaxWidth(),
+            ) {
+                if (state.isChecking) {
+                    CircularProgressIndicator(
+                        modifier = Modifier.size(16.dp),
+                        strokeWidth = 2.dp,
+                        color = MaterialTheme.colorScheme.onPrimary,
+                    )
+                    Spacer(Modifier.size(8.dp))
+                }
+                Text(if (state.isChecking) "Checking…" else "Check for updates")
+            }
         }
     }
 }
 
+@Composable
+private fun UpdateCheckLine(result: UpdateCheckResult) {
+    val text = when (result) {
+        UpdateCheckResult.Idle -> return
+        UpdateCheckResult.Latest -> "You're on the latest version."
+        is UpdateCheckResult.UpdateAvailable ->
+            "Update available: ${result.info.version}"
+        is UpdateCheckResult.Error -> "Couldn't check: ${result.message}"
+    }
+    Text(
+        text = text,
+        style = MaterialTheme.typography.bodySmall,
+        color = MaterialTheme.colorScheme.onSurfaceVariant,
+    )
+}
+
 @Composable
 private fun SignOutButton(isSigningOut: Boolean, onSignOut: () -> Unit) {
     Button(

android/app/src/main/java/com/fabledsword/minstrel/update/api/ClientVersionApi.kt

@@ -0,0 +1,14 @@
+package com.fabledsword.minstrel.update.api
+
+import com.fabledsword.minstrel.models.wire.UpdateInfoWire
+import retrofit2.http.GET
+
+/**
+ * Retrofit interface for `GET /api/client/version`. Returns the
+ * server-bundled APK version + download URL + size. Mirrors the
+ * Flutter `ClientUpdateController` shape.
+ */
+interface ClientVersionApi {
+    @GET("api/client/version")
+    suspend fun getVersion(): UpdateInfoWire
+}

android/app/src/main/java/com/fabledsword/minstrel/update/data/UpdateRepository.kt

@@ -0,0 +1,61 @@
+package com.fabledsword.minstrel.update.data
+
+import com.fabledsword.minstrel.models.UpdateInfo
+import com.fabledsword.minstrel.models.wire.UpdateInfoWire
+import com.fabledsword.minstrel.update.api.ClientVersionApi
+import retrofit2.Retrofit
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Thin facade over `/api/client/version`. Reused by the About card
+ * (Check for updates button) and — in a follow-up — a startup
+ * version banner.
+ */
+@Singleton
+class UpdateRepository @Inject constructor(retrofit: Retrofit) {
+    private val api: ClientVersionApi = retrofit.create(ClientVersionApi::class.java)
+
+    suspend fun getLatest(): UpdateInfo = api.getVersion().toDomain()
+}
+
+private fun UpdateInfoWire.toDomain(): UpdateInfo = UpdateInfo(
+    version = version,
+    apkUrl = apkUrl,
+    sizeBytes = sizeBytes,
+)
+
+/**
+ * True when [server] is strictly newer than [installed]. Mirrors
+ * Flutter's `isVersionNewer` — splits both strings on `.`, parses
+ * each component as an int (non-numeric = 0), pads the shorter list
+ * with zeros, then compares component-wise. Handles our date-style
+ * versions (2026.05.10.1) which exceed semver's three-part shape,
+ * and treats "2026.05.10" as equal to "2026.05.10.0".
+ *
+ * Falls back to string inequality when neither side parsed any
+ * non-zero component (branch-name builds like "main" vs "dev") so
+ * a dev build still gets the banner.
+ */
+fun isVersionNewer(server: String, installed: String): Boolean {
+    val svr = parseVersion(server)
+    val ins = parseVersion(installed)
+    if (svr.all { it == 0 } && ins.all { it == 0 }) {
+        return server.removePrefix("v") != installed.removePrefix("v")
+    }
+    val n = maxOf(svr.size, ins.size)
+    val svrPadded = svr + List(n - svr.size) { 0 }
+    val insPadded = ins + List(n - ins.size) { 0 }
+    return compareComponentWise(svrPadded, insPadded, n)
+}
+
+private fun parseVersion(v: String): List<Int> =
+    v.removePrefix("v").split('.').map { it.toIntOrNull() ?: 0 }
+
+private fun compareComponentWise(svr: List<Int>, ins: List<Int>, n: Int): Boolean {
+    for (i in 0 until n) {
+        if (svr[i] > ins[i]) return true
+        if (svr[i] < ins[i]) return false
+    }
+    return false
+}