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/lib/update/installer.dart

@@ -0,0 +1,46 @@
+import 'package:dio/dio.dart';
+import 'package:flutter/services.dart';
+import 'package:path_provider/path_provider.dart';
+
+/// Bridges to MainActivity.kt's MethodChannel for the Android
+/// PackageInstaller intent (#397). Web / iOS don't support self-
+/// install; this class is Android-only at v1.
+class UpdateInstaller {
+  UpdateInstaller(this._dio);
+  final Dio _dio;
+
+  static const _channel = MethodChannel('com.fabledsword.minstrel/installer');
+  static const _filename = 'minstrel-update.apk';
+
+  /// Streams the APK from `apkUrl` into the cache directory. `onProgress`
+  /// receives 0..1 fractions; emits 1.0 once when the download completes.
+  /// Returns the local path the install intent will read from.
+  Future<String> download(
+    String apkUrl, {
+    void Function(double progress)? onProgress,
+  }) async {
+    final cacheDir = await getApplicationCacheDirectory();
+    final path = '${cacheDir.path}/$_filename';
+    await _dio.download(
+      apkUrl,
+      path,
+      onReceiveProgress: (received, total) {
+        if (total > 0 && onProgress != null) {
+          onProgress(received / total);
+        }
+      },
+    );
+    return path;
+  }
+
+  /// Hands the downloaded APK to Android's PackageInstaller via a
+  /// FileProvider content:// URI. The system shows the install confirm
+  /// dialog; user must tap Install. App restarts on the new version.
+  ///
+  /// First-ever install attempt prompts the user to flip "Install
+  /// unknown apps" for Minstrel in Settings → Apps → Special access.
+  /// One-time grant; persists across updates.
+  Future<void> install(String apkPath) async {
+    await _channel.invokeMethod('install', {'path': apkPath});
+  }
+}