feat: background update download, snackbar prompt, chat image rendering

Update provider: auto-downloads APK in background after finding a newer
version, prompts via snackbar only when ready, cleans up old APKs on
startup. Replaces modal dialog with dismissible snackbar.

Chat bubble: resolve relative image URLs (/api/images/{id}) against
server base URL with auth cookies so search_images results render on
the phone app.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/screens/settings/settings_screen.dart

@@ -92,7 +92,7 @@ class SettingsScreen extends ConsumerWidget {
                     child: const Text('Check'),
                   ),
           ),
-          if (update.status == UpdateStatus.available ||
+          if (update.status == UpdateStatus.readyToInstall ||
               update.status == UpdateStatus.downloading)
             _UpdateTile(update: update),
           if (update.status == UpdateStatus.error)
@@ -131,7 +131,7 @@ class SettingsScreen extends ConsumerWidget {
     if (update.status == UpdateStatus.upToDate) {
       return Text('v$current — up to date');
     }
-    if (update.status == UpdateStatus.available ||
+    if (update.status == UpdateStatus.readyToInstall ||
         update.status == UpdateStatus.downloading) {
       return Text('v$current installed');
     }
@@ -202,12 +202,12 @@ class _UpdateTile extends ConsumerWidget {
                     '${(update.downloadProgress * 100).toStringAsFixed(0)}%'),
               ],
             )
-          : const Text('Tap to download and install'),
+          : const Text('Ready to install'),
       trailing: isDownloading
           ? null
           : FilledButton(
               onPressed: () =>
-                  ref.read(updateProvider.notifier).downloadAndInstall(),
+                  ref.read(updateProvider.notifier).install(),
               child: const Text('Install'),
             ),
     );