[mob][photos] Prevent vectorDB index file corruption (#7049)

## Description

- Use `load` instead of `view`, since latter is read-only
- When loading fails in rust, delete index file in dart side and try
again
- Atomically save index file by first writing to temp file

## Tests

Tested in debug mode on my pixel phone.

mobile/apps/photos/lib/db/ml/clip_vector_db.dart

@@ -39,10 +39,26 @@ class ClipVectorDB {
     final documentsDirectory = await getApplicationDocumentsDirectory();
     final String dbPath = join(documentsDirectory.path, _databaseName);
     _logger.info("Opening vectorDB access: DB path " + dbPath);
-    final vectorDB = VectorDb(
-      filePath: dbPath,
-      dimensions: _embeddingDimension,
-    );
+    late VectorDb vectorDB;
+    try {
+      vectorDB = VectorDb(
+        filePath: dbPath,
+        dimensions: _embeddingDimension,
+      );
+    } catch (e, s) {
+      _logger.severe("Could not open VectorDB at path $dbPath", e, s);
+      _logger.severe("Deleting the index file and trying again");
+      await deleteIndexFile();
+      try {
+        vectorDB = VectorDb(
+          filePath: dbPath,
+          dimensions: _embeddingDimension,
+        );
+      } catch (e, s) {
+        _logger.severe("Still can't open VectorDB at path $dbPath", e, s);
+        rethrow;
+      }
+    }
     final stats = await getIndexStats(vectorDB);
     _logger.info("VectorDB connection opened with stats: ${stats.toString()}");
 

mobile/apps/photos/lib/ui/tools/similar_images_page.dart

@@ -672,10 +672,7 @@ class _SimilarImagesPageState extends State<SimilarImagesPage>
         await showGenericErrorDialog(context: context, error: e);
       }
       if (_isDisposed) return;
-      setState(() {
-        _pageState = SimilarImagesPageState.setup;
-      });
-      return;
+      Navigator.of(context).pop();
     }
   }
 

mobile/apps/photos/rust/src/api/usearch_api.rs

@@ -32,8 +32,11 @@ impl VectorDB {
 
         if file_exists {
             println!("Loading index from disk.");
-            // Use view to not load the index into memory. https://docs.rs/usearch/latest/usearch/struct.Index.html#method.view
-            db.index.view(file_path).expect("Failed to load index");
+            // Must use load() instead of view() because:
+            // - view() creates a read-only memory-mapped view (immutable)
+            // - load() loads the index into RAM for read/write operations (mutable)
+            // Using view() causes "Can't add to an immutable index" error
+            db.index.load(file_path).expect("Failed to load index");
         } else {
             println!("Creating new index.");
             db.save_index();
@@ -46,9 +49,37 @@ impl VectorDB {
         if let Some(parent) = self.path.parent() {
             std::fs::create_dir_all(parent).expect("Failed to create directory");
         }
-        self.index
-            .save(self.path.to_str().expect("Invalid path"))
-            .expect("Failed to save index");
+
+        // Use atomic write: save to temp file first, then rename
+        let temp_path = self.path.with_extension("tmp");
+        let temp_path_str = temp_path.to_str().expect("Invalid temp path");
+
+        // Save to temporary file
+        match self.index.save(temp_path_str) {
+            Ok(_) => {
+                // Atomic rename - guaranteed atomic on iOS/Android
+                // This will atomically replace the existing file
+                // The rename ensures we never have a partially written file,
+                // even if the app is suspended or crashes
+                match std::fs::rename(&temp_path, &self.path) {
+                    Ok(_) => {
+                        println!("Successfully saved index atomically");
+                    }
+                    Err(e) => {
+                        println!("Failed to rename temp index file: {:?}", e);
+                        // Try to clean up temp file
+                        let _ = std::fs::remove_file(&temp_path);
+                        panic!("Failed to atomically save index: {:?}", e);
+                    }
+                }
+            }
+            Err(e) => {
+                println!("Failed to save index to temp file: {:?}", e);
+                // Try to clean up temp file if it exists
+                let _ = std::fs::remove_file(&temp_path);
+                panic!("Failed to save index: {:?}", e);
+            }
+        }
     }
 
     fn ensure_capacity(&self, margin: usize) {