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-tauri/src/mobile.rs

@@ -0,0 +1,33 @@
+use tauri::command;
+
+#[command]
+pub async fn open_file_with_provider(path: String, mime_type: Option<String>) -> Result<(), String> {
+    #[cfg(target_os = "android")]
+    {
+        open_file_android(path, mime_type).await
+    }
+
+    #[cfg(not(target_os = "android"))]
+    {
+        // On other platforms, use the opener plugin
+        use tauri_plugin_opener::OpenerExt;
+        tauri::AppHandle::opener()
+            .open_path(path, None::<&str>)
+            .map_err(|e| e.to_string())
+    }
+}
+
+#[cfg(target_os = "android")]
+async fn open_file_android(path: String, mime_type: Option<String>) -> Result<(), String> {
+    use jni::{
+        objects::{JObject, JString, JValue},
+        JNIEnv,
+    };
+    use tauri::Manager;
+
+    // This will be called from the Rust side
+    // We need to call into the Android Activity to use FileProvider
+
+    // For now, return an error - we need to implement the Java/Kotlin side first
+    Err("Android FileProvider implementation needed".to_string())
+}

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[]