feat: Implement fullscreen, keyboard controls, and pulsating heptagon

This commit fulfills tasks 1 and 2, and adds a synchronized visual effect.

- **Fullscreen Mode**: Added '--fullscreen' command-line argument and dynamic toggling via 'F' key.

- **Keyboard Controls**: Implemented 'Esc' to exit and 'F' to toggle fullscreen in 'src/platform.cc'.

- **Synchronized Visuals**: Added a pulsating heptagon effect in 'src/gpu/gpu.cc' and 'src/gpu/shader.wgsl' that scales and changes color based on the real-time audio peak from the synth.

- **Refactor**: Abstracted platform-specific WebGPU surface creation into 'src/platform.cc' to keep 'src/gpu/gpu.cc' cross-platform.

- **Build System**: Corrected 'CMakeLists.txt' to properly link 'wgpu-native' and platform frameworks, and updated 'project_init.sh' to build the submodule.

- **Documentation**: Updated 'HOWTO.md' and 'PROJECT_CONTEXT.md' with new features and decisions.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/audio/synth.h b/src/audio/synth.h
index eadde10..fe28e8d 100644
--- a/src/audio/synth.h
+++ b/src/audio/synth.h
@@ -23,3 +23,4 @@ void synth_commit_update(int spectrogram_id);
 void synth_trigger_voice(int spectrogram_id, float volume, float pan);
 void synth_render(float *output_buffer, int num_frames);
 int synth_get_active_voice_count();
+float synth_get_output_peak();