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/audio/Audio.h

@@ -43,6 +43,12 @@ public:
     void shuffle();               // randomize order
     void start();                 // begin playback
     void toggleMute();
+    
+    // Menu music support
+    void setMenuTrack(const std::string& path);
+    void playMenuMusic();
+    void playGameMusic();
+    
     // Queue a sound effect to mix over the music (pcm can be mono/stereo, any rate; will be converted)
     void playSfx(const std::vector<int16_t>& pcm, int channels, int rate, float volume);
     void shutdown();
@@ -54,7 +60,10 @@ private:
     bool ensureStream();
     void backgroundLoadingThread(); // background thread function
     
-    std::vector<AudioTrack> tracks; int current=-1; bool playing=false; bool muted=false; std::mt19937 rng{std::random_device{}()};
+    std::vector<AudioTrack> tracks; 
+    AudioTrack menuTrack;
+    bool isMenuMusic = false;
+    int current=-1; bool playing=false; bool muted=false; std::mt19937 rng{std::random_device{}()};
     SDL_AudioStream* audioStream=nullptr; SDL_AudioSpec outSpec{}; int outChannels=2; int outRate=44100; bool mfStarted=false;
     
     // Threading support