feat(flutter): in-app update banner + Android install intent (#397 phase 2)

Closes the operator-facing half of #397. Soft banner mounts at the
top of the shell when the server has a newer APK bundled at
/api/client/version; tap Install → APK downloads to cache → Android
PackageInstaller intent fires → user taps Install in the system
dialog → app restarts on the new version.

### Dart side

- lib/update/update_info.dart — wire shape for /api/client/version
- lib/update/client_update_provider.dart — Riverpod AsyncNotifier
  polling on app start + every 24h. Compares semver via pub_semver
  with non-semver string-equality fallback. Server 404 = "no update
  channel" = silent. Companion shouldShowUpdateBannerProvider gates
  on a per-version dismissed-set (in-memory; resets on restart).
- lib/update/installer.dart — UpdateInstaller wraps dio.download +
  the MethodChannel call to MainActivity.kt.
- lib/update/update_banner.dart — Material banner with idle /
  downloading (with progress) / error stages. Mounted in
  _ShellWithPlayerBar above the route content; SizedBox.shrink()
  when no update available so non-shell routes (login, server-url)
  see no banner anyway.
- isVersionNewer() extracted as a pure function and tested across
  semver, prerelease ordering, leading-v normalization, and
  non-semver fallback (date-tag style).

### Android side

- AndroidManifest.xml — REQUEST_INSTALL_PACKAGES permission;
  FileProvider with authorities ${applicationId}.fileprovider.
- res/xml/file_paths.xml — exposes the cache directory so the
  downloaded APK can be served via content:// URI (file:// is
  blocked since Android 7).
- MainActivity.kt — MethodChannel handler builds the FileProvider
  URI and fires Intent.ACTION_VIEW with FLAG_GRANT_READ_URI_PERMISSION
  so the system installer can read across the process boundary.

Phase 3 (CI sequencing — bake APK + version file into /app/client/
during release.yml's tag flow) is the remaining piece. Without it
the server returns 404 from /api/client/version and the banner
silently does nothing — graceful degradation, but no actual updates.

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

flutter_client/android/app/src/main/AndroidManifest.xml

@@ -9,6 +9,10 @@
          for the media notification (now-playing tile) to appear. Audio
          playback works without it; only the system-tray surface breaks. -->
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+    <!-- In-app update flow (#397). First update attempt prompts the
+         user to enable "Install unknown apps" for Minstrel in
+         Settings → Apps → Special access. One-time grant; persists. -->
+    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
     <application
         android:label="minstrel"
         android:name="${applicationName}"
@@ -56,6 +60,20 @@
                 <action android:name="android.intent.action.MEDIA_BUTTON" />
             </intent-filter>
         </receiver>
+        <!-- FileProvider for the in-app update flow (#397). Maps the
+             cache directory (where dio.download writes the APK) to a
+             content:// URI so the PackageInstaller intent can read it
+             across the process boundary. file:// URIs are blocked
+             since Android 7. -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths" />
+        </provider>
     </application>
     <!-- Required to query activities that can process text, see:
          https://developer.android.com/training/package-visibility and

flutter_client/android/app/src/main/kotlin/com/fabledsword/minstrel/MainActivity.kt

@@ -1,6 +1,11 @@
 package com.fabledsword.minstrel
 
+import android.content.Intent
+import androidx.core.content.FileProvider
 import com.ryanheise.audioservice.AudioServiceActivity
+import io.flutter.embedding.engine.FlutterEngine
+import io.flutter.plugin.common.MethodChannel
+import java.io.File
 
 // Must extend AudioServiceActivity (not FlutterActivity) so the
 // audio_service plugin can wire its FlutterEngine through. Without this
@@ -8,4 +13,58 @@ import com.ryanheise.audioservice.AudioServiceActivity
 // "The Activity class declared in your AndroidManifest.xml is wrong",
 // which on first start cascades into a flutter_cache_manager sqlite
 // EXCLUSIVE-lock crash because the engine partially re-initialises.
-class MainActivity : AudioServiceActivity()
+class MainActivity : AudioServiceActivity() {
+
+    companion object {
+        private const val INSTALLER_CHANNEL = "com.fabledsword.minstrel/installer"
+    }
+
+    override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
+        super.configureFlutterEngine(flutterEngine)
+
+        // In-app update channel (#397). Dart calls install(path) with
+        // the cache-dir APK path; we hand it to Android's
+        // PackageInstaller via FileProvider + ACTION_VIEW.
+        MethodChannel(flutterEngine.dartExecutor.binaryMessenger, INSTALLER_CHANNEL)
+            .setMethodCallHandler { call, result ->
+                when (call.method) {
+                    "install" -> {
+                        val path = call.argument<String>("path")
+                        if (path == null) {
+                            result.error("missing_path", "path argument required", null)
+                            return@setMethodCallHandler
+                        }
+                        try {
+                            installApk(path)
+                            result.success(null)
+                        } catch (e: Exception) {
+                            result.error("install_failed", e.message, null)
+                        }
+                    }
+                    else -> result.notImplemented()
+                }
+            }
+    }
+
+    private fun installApk(path: String) {
+        val file = File(path)
+        if (!file.exists()) {
+            throw IllegalStateException("apk not found at $path")
+        }
+        val uri = FileProvider.getUriForFile(
+            this,
+            "${packageName}.fileprovider",
+            file
+        )
+        val intent = Intent(Intent.ACTION_VIEW).apply {
+            setDataAndType(uri, "application/vnd.android.package-archive")
+            // NEW_TASK: PackageInstaller runs in its own task.
+            // GRANT_READ_URI_PERMISSION: hands the content:// URI to
+            // the installer process, which lacks our app's read perms
+            // by default.
+            flags = Intent.FLAG_ACTIVITY_NEW_TASK or
+                Intent.FLAG_GRANT_READ_URI_PERMISSION
+        }
+        startActivity(intent)
+    }
+}

flutter_client/android/app/src/main/res/xml/file_paths.xml

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+  FileProvider mappings for the in-app update flow (#397).
+  - cache-path/. exposes the app's getCacheDir() so the downloaded
+    APK at <cache>/minstrel-update.apk can be read by the
+    PackageInstaller via the content:// URI built in MainActivity.kt.
+-->
+<paths xmlns:android="http://schemas.android.com/apk/res/android">
+    <cache-path name="updates" path="." />
+</paths>