feat(android): connectivity observer + shell ConnectionErrorBanner (audit v2 #21)

Adds the foundational network-state plumbing the audit called out:

* ConnectivityObserver — Hilt singleton wrapping ConnectivityManager.
  Exposes a Flow<Boolean> sourced from registerNetworkCallback; emits
  false when the active network lacks INTERNET+VALIDATED capabilities
  (airplane mode, no carrier, captive portal) and true once a usable
  network appears. Seeded with the initial value so the banner doesn't
  flash before the first capability callback.

* ConnectionErrorBanner — shell-level Compose banner that AnimatedVisibility-
  shrinks/expands based on the observer. Red errorContainer surface,
  CloudOff icon, "No connection — check Wi-Fi or mobile data." copy.
  Owns a tiny ConnectivityBannerViewModel that lifts the singleton's
  Flow into a lifecycle-scoped StateFlow.

* ShellScaffold now invokes ConnectionErrorBanner() in the banner
  slot above the routed content. VersionTooOld / UpdateBanner will
  join the same slot in follow-up commits.

ACCESS_NETWORK_STATE permission was already in the manifest. Downstream
repositories can also collect ConnectivityObserver.online to gate
retry loops once that wiring is needed.

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

android/app/src/main/java/com/fabledsword/minstrel/connectivity/ConnectivityObserver.kt

@@ -0,0 +1,70 @@
+package com.fabledsword.minstrel.connectivity
+
+import android.content.Context
+import android.net.ConnectivityManager
+import android.net.Network
+import android.net.NetworkCapabilities
+import android.net.NetworkRequest
+import dagger.hilt.android.qualifiers.ApplicationContext
+import kotlinx.coroutines.channels.awaitClose
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.callbackFlow
+import kotlinx.coroutines.flow.distinctUntilChanged
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * Single source of truth for the device's "is the internet usable
+ * right now" signal — wraps [ConnectivityManager] and exposes a hot
+ * cold-startable Flow that emits `false` while the active network
+ * lacks INTERNET + VALIDATED capabilities (airplane mode, no carrier,
+ * captive portal, etc.) and `true` once a usable network appears.
+ *
+ * Used by the shell-level ConnectionErrorBanner; downstream
+ * repositories can also collect this to gate retry loops.
+ */
+@Singleton
+class ConnectivityObserver @Inject constructor(
+    @ApplicationContext context: Context,
+) {
+    private val cm = context.getSystemService(ConnectivityManager::class.java)
+
+    val online: Flow<Boolean> = callbackFlow {
+        val request = NetworkRequest.Builder()
+            .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
+            .build()
+        val callback = object : ConnectivityManager.NetworkCallback() {
+            override fun onAvailable(network: Network) {
+                trySend(hasUsableInternet())
+            }
+            override fun onLost(network: Network) {
+                trySend(hasUsableInternet())
+            }
+            override fun onCapabilitiesChanged(
+                network: Network,
+                capabilities: NetworkCapabilities,
+            ) {
+                trySend(
+                    capabilities.hasCapability(
+                        NetworkCapabilities.NET_CAPABILITY_INTERNET,
+                    ) &&
+                        capabilities.hasCapability(
+                            NetworkCapabilities.NET_CAPABILITY_VALIDATED,
+                        ),
+                )
+            }
+        }
+        cm.registerNetworkCallback(request, callback)
+        // Seed the initial value so the banner doesn't flash before the
+        // first capability callback fires.
+        trySend(hasUsableInternet())
+        awaitClose { cm.unregisterNetworkCallback(callback) }
+    }.distinctUntilChanged()
+
+    private fun hasUsableInternet(): Boolean {
+        val active = cm.activeNetwork ?: return false
+        val caps = cm.getNetworkCapabilities(active) ?: return false
+        return caps.hasCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET) &&
+            caps.hasCapability(NetworkCapabilities.NET_CAPABILITY_VALIDATED)
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/connectivity/ui/ConnectionErrorBanner.kt

@@ -0,0 +1,90 @@
+package com.fabledsword.minstrel.connectivity.ui
+
+import androidx.compose.animation.AnimatedVisibility
+import androidx.compose.animation.expandVertically
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.shrinkVertically
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.Icon
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.unit.dp
+import androidx.hilt.navigation.compose.hiltViewModel
+import androidx.lifecycle.SavedStateHandle
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.compose.collectAsStateWithLifecycle
+import androidx.lifecycle.viewModelScope
+import com.composables.icons.lucide.CloudOff
+import com.composables.icons.lucide.Lucide
+import com.fabledsword.minstrel.connectivity.ConnectivityObserver
+import dagger.hilt.android.lifecycle.HiltViewModel
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.stateIn
+import javax.inject.Inject
+
+private const val ONLINE_SHARE_STOP_TIMEOUT_MS = 5_000L
+
+/**
+ * Tiny VM that just lifts the [ConnectivityObserver] singleton's
+ * Flow into a StateFlow with the standard sharing strategy. Keeps
+ * the banner composable pure-presentation.
+ */
+@HiltViewModel
+class ConnectivityBannerViewModel @Inject constructor(
+    observer: ConnectivityObserver,
+    @Suppress("UnusedPrivateProperty") savedStateHandle: SavedStateHandle,
+) : ViewModel() {
+    val online: StateFlow<Boolean> = observer.online.stateIn(
+        scope = viewModelScope,
+        started = SharingStarted.WhileSubscribed(ONLINE_SHARE_STOP_TIMEOUT_MS),
+        initialValue = true,
+    )
+}
+
+/**
+ * Banner shown at the top of the shell when the device has no usable
+ * internet. Mirrors Flutter's ConnectionErrorBanner: red-tinted error
+ * surface, CloudOff icon, "No connection — check Wi-Fi or mobile
+ * data" copy. Auto-hides via slide+fade when connectivity returns.
+ */
+@Composable
+fun ConnectionErrorBanner(
+    viewModel: ConnectivityBannerViewModel = hiltViewModel(),
+) {
+    val online by viewModel.online.collectAsStateWithLifecycle()
+    AnimatedVisibility(
+        visible = !online,
+        enter = expandVertically() + fadeIn(),
+        exit = shrinkVertically() + fadeOut(),
+    ) {
+        Row(
+            modifier = Modifier
+                .fillMaxWidth()
+                .background(MaterialTheme.colorScheme.errorContainer)
+                .padding(horizontal = 16.dp, vertical = 10.dp),
+            verticalAlignment = Alignment.CenterVertically,
+            horizontalArrangement = Arrangement.spacedBy(12.dp),
+        ) {
+            Icon(
+                imageVector = Lucide.CloudOff,
+                contentDescription = null,
+                tint = MaterialTheme.colorScheme.onErrorContainer,
+            )
+            Text(
+                text = "No connection — check Wi-Fi or mobile data.",
+                style = MaterialTheme.typography.bodyMedium,
+                color = MaterialTheme.colorScheme.onErrorContainer,
+            )
+        }
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/shared/widgets/ShellScaffold.kt

@@ -12,6 +12,7 @@ import androidx.compose.runtime.remember
 import androidx.compose.ui.Modifier
 import androidx.hilt.navigation.compose.hiltViewModel
 import androidx.navigation.NavHostController
+import com.fabledsword.minstrel.connectivity.ui.ConnectionErrorBanner
 import com.fabledsword.minstrel.nav.AlbumDetail
 import com.fabledsword.minstrel.nav.ArtistDetail
 import com.fabledsword.minstrel.player.ui.MiniPlayer
@@ -52,9 +53,10 @@ fun ShellScaffold(
         }
     }
     Column(modifier = modifier.fillMaxSize()) {
-        // Banners (VersionTooOld, UpdateBanner) live here once those
-        // mechanisms exist; for now the slot is empty so the layout
-        // matches the Flutter structure 1:1.
+        // Banner slot. ConnectionErrorBanner auto-shows when the device
+        // loses usable internet. VersionTooOld / UpdateBanner will join
+        // this slot once those mechanisms exist.
+        ConnectionErrorBanner()
         Box(modifier = Modifier.fillMaxWidth().weight(1f)) {
             content()
         }