Add SIDScore spectrogram visualisation

[turesheim]

Summary

• add a per-voice spectrogram mode alongside the stabilised waveform view
• analyse raw SIDScore samples with overlapping Hann-windowed FFT frames and a logarithmic frequency axis
• use a fixed -96 to 0 dBFS colour scale with frequency and time guides
• cache each voice analysis until its sample buffer changes
• add focused tests for frequency placement, time direction, level scaling and silence
• document the Waveform and Spectrogram views, their scales and their interpretation in the README
• add current 1440x900 product screenshots for both visualisation modes

Testing

• npm test --workspace @commodore-commander/theia-extension with Java 21 (153 tests passed)
• npm run theia:build with Java 21
• Electron bundle build
• live Electron README captures at 1440x900 with all three voices playing
• visual checks for the common waveform trigger, non-empty per-voice spectrograms, axes, legend and layout