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