feat(android): in-app APK update install (audit v3 #25)

Wires the "Install vX.Y.Z" action the About card's update-check
teed up.

* ApkInstaller @Singleton — downloads the server APK via the shared
  OkHttpClient (inherits auth cookie + BaseUrlInterceptor host
  rewrite; apkUrl is server-relative) into cacheDir, then launches
  the system installer via a FileProvider content:// URI. canInstall()
  gates on PackageManager.canRequestPackageInstalls() on O+, and
  requestInstallPermission() opens the "install unknown apps" settings
  page when not yet granted.
* Manifest: REQUEST_INSTALL_PACKAGES permission + FileProvider
  (${applicationId}.fileprovider) + res/xml/file_paths.xml exposing
  the cache dir.
* AboutCardViewModel.install(info): permission check → download →
  launch, with isInstalling + installMessage state. Errors routed
  through ErrorCopy.
* About card shows an "Install vX.Y.Z" button under the check button
  when an update is available, "Downloading…" while in flight, and
  the install message line. Extracted UpdateControls / InstallButton /
  ButtonSpinner helpers to keep AboutCard under the length cap;
  added @file:Suppress(TooManyFunctions) for the settings-card density.

Closes audit v3 #25 — the last open parity item from the v3 sweep
aside from the offline-pool Home cards (#28).

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

android/app/src/main/AndroidManifest.xml

@@ -8,6 +8,7 @@
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK" />
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+    <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
 
     <application
         android:name=".MinstrelApplication"
@@ -39,5 +40,15 @@
                 <action android:name="androidx.media3.session.MediaSessionService" />
             </intent-filter>
         </service>
+
+        <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>
 </manifest>