feat: fit-contain letterbox + smart queue refill

- Replace center-crop with fit-contain (no photo cropping)
- Letterbox bars filled with adjacent edge average color
- Queue fetches in batches until 50 usable assets
- Auto-refill queue on exhaustion, dedup via shown history
- Filter RAW files from asset selection

Co-authored-by: Cursor <cursoragent@cursor.com>

src/photo_queue.h

@@ -22,8 +22,17 @@ private:
     SettingsManager* _settings = nullptr;
 
     std::vector<String> _queue;
+    std::vector<String> _shown;  // Recently shown IDs for dedup on refill
     size_t _cursor = 0;
     unsigned long _lastSyncTime = 0;
+    unsigned long _lastSyncAttempt = 0;
+    unsigned long _syncRetryDelay = 10000;
+
+    static constexpr unsigned long SYNC_RETRY_MIN = 10000;     // 10 seconds
+    static constexpr unsigned long SYNC_RETRY_MAX = 300000;    // 5 minutes
+    static constexpr size_t TARGET_QUEUE_SIZE = 50;
+    static constexpr int MAX_FETCH_ROUNDS = 5;
+    static constexpr size_t MAX_SHOWN_HISTORY = 200;
 
     void shuffle();
     void sortChronological(bool reverse);