feat: Add Firebase high score sync, menu music, and gameplay improvements

- Integrate Firebase Realtime Database for high score synchronization
  - Add cpr and nlohmann-json dependencies for HTTP requests
  - Implement async score loading from Firebase with local fallback
  - Submit all scores > 0 to Firebase in background thread
  - Always prompt for player name on game over if score > 0

- Add dedicated menu music system
  - Implement menu track support in Audio class with looping
  - Add "Every Block You Take.mp3" as main menu theme
  - Automatically switch between menu and game music on state transitions
  - Load menu track asynchronously to prevent startup delays

- Update level speed progression to match web version
  - Replace NES frame-based gravity with explicit millisecond values
  - Implement 20-level speed table (1000ms to 60ms)
  - Ensure consistent gameplay between C++ and web versions

- Fix startup performance issues
  - Move score loading to background thread to prevent UI freeze
  - Optimize Firebase network requests with 2s timeout
  - Add graceful fallback to local scores on network failure

Files modified:
- src/persistence/Scores.cpp/h - Firebase integration
- src/audio/Audio.cpp/h - Menu music support
- src/core/GravityManager.cpp/h - Level speed updates
- src/main.cpp - State-based music switching, async loading
- CMakeLists.txt - Add cpr and nlohmann-json dependencies
- vcpkg.json - Update dependency list

src/core/GravityManager.h

@@ -18,14 +18,11 @@ public:
     double getFpsForLevel(int level) const;
 
 private:
-    static constexpr double NES_FPS = 60.0988;
-    static constexpr double FRAME_MS = 1000.0 / NES_FPS;
-    static constexpr int FRAMES_TABLE[30] = {
-        48,43,38,33,28,23,18,13,8,6,
-        5,5,5,4,4,4,3,3,3,2,
-        2,2,2,2,2,2,2,2,2,1
+    static constexpr double LEVEL_SPEEDS_MS[20] = {
+        1000.0, 920.0, 840.0, 760.0, 680.0, 600.0, 520.0, 440.0, 360.0, 280.0,
+        200.0,  160.0, 160.0, 120.0, 120.0, 100.0, 100.0, 80.0,  80.0,  60.0
     };
 
     double globalMultiplier{1.0};
-    std::array<double,30> levelMultipliers{}; // default 1.0
+    std::array<double,20> levelMultipliers{}; // default 1.0
 };