fix files directory handling: use internal storage and add exception handling

- Use internal files dir instead of external (fixes scoped storage permissions)
- Pass files directory from Android context via JNI instead of hardcoding
- Add exception handling in ScanAppsDirectory to prevent crashes on permission errors
- Use std::error_code overload of fs::exists() to avoid throwing on access denial

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/main/java/com/omixlab/mosis/NativeService.kt

@@ -19,6 +19,7 @@ class NativeService : Service() {
     }
     external fun getBinderNative(): IBinder
     external fun setAssetManager(assetManager: AssetManager)
+    external fun setFilesDir(filesDir: String)
 
     override fun onBind(intent: Intent): IBinder {
         return getBinderNative()
@@ -27,6 +28,9 @@ class NativeService : Service() {
     override fun onCreate() {
         super.onCreate()
         setAssetManager(assets)
+        // Use internal files dir - more reliable access in service processes
+        // For debugging, use: adb shell run-as com.omixlab.mosis ls files/
+        setFilesDir(filesDir.absolutePath)
     }
 
     override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {