Add wavesurfer.js waveform visualization to sample browser

[LukeMainwaring]

Summary

• Add interactive waveform visualization to the sample browser sidebar using wavesurfer.js
• Replace raw HTMLAudioElement playback with a WaveformViz component that shows a seekable waveform with progress and time display
• Waveform renders only for the currently playing sample to avoid fetching/decoding audio for every card

Changes

• frontend/components/waveform-viz.tsx (new) — reusable wavesurfer.js wrapper with theme-aware colors, autoplay, seek-by-click, and time display
• frontend/components/sample-browser.tsx — replaced new Audio() / audioRef pattern with WaveformViz; card expands to show waveform when playing
• frontend/package.json — added wavesurfer.js@7.12.4 and @wavesurfer/react@1.0.12

Test Plan

• Click play on a sample card → waveform appears below metadata, audio plays
• Click the waveform to seek → playback jumps to that position
• Click play on a different card → previous waveform disappears, new one appears
• Let a sample finish → waveform disappears, card returns to normal
• Click pause on playing sample → playback stops, waveform disappears
• Verify waveform colors work in both light and dark themes

🤖 Generated with Claude Code