Move all Unix-specific syscalls (fork, execvp, waitpid, strerror, /tmp)
into a platform layer so the same spectrogram.c can target Windows
(CreateProcess) and WebAssembly (stubs). Key changes:
- src/platform.h: public API with spawn handle, error codes, path helpers
- src/platform_linux.c: fork/execvp implementation, /tmp, strerror
- src/platform_win32.c: CreateProcess implementation, temp dir lookup
- src/platform_web.c: stub implementations (no subprocess support on web)
- src/spectrogram.c: consume only platform.h; replace pid_t/fork/execvp
with Platform_SpawnChild/WaitForChild; use Platform_GetTempDir() for
/tmp path; replace strdup with malloc/memcpy for portability
- Build: add platform_*.c to gmake, Premake, and build_web.sh
- Add incremental STFT processing with loading overlay and progress bar
- Add auto-scaling amplitude (max dB, max-40dB floor) for low-signal files
- Replace fixed bitmap text with TTF font scaled by window size for crisp rendering at any resolution
- Fix command-line file path resolution relative to original working directory
- Scale file browser dialog dimensions by GetUIScale() to prevent text overlap
- Disable ESC key closing the window
- Fix FFT size changes to restart STFT computation
tyler