Add showImage handler stub and mobile file provider foundation

- Add haextension.fs.showImage handler that delegates to frontend PhotoSwipe
- Add mobile.rs with open_file_with_provider command for future Android FileProvider integration
- Keep showImage as backwards-compatible no-op since image viewing is now handled client-side

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/composables/handlers/filesystem.ts

@@ -44,6 +44,15 @@ export async function handleFilesystemMethodAsync(
       }
     }
 
+    case 'haextension.fs.showImage': {
+      // This method is now handled by the frontend using PhotoSwipe
+      // We keep it for backwards compatibility but it's a no-op
+      return {
+        success: true,
+        useFrontend: true,
+      }
+    }
+
     case 'haextension.fs.openFile': {
       const params = request.params as {
         data: number[]