refactor(sound): extract playActionSound from FileViewer into src/sound.ts

[killerwolf]

Summary

• FileViewer.tsx had a ~105-line inline Web Audio synthesis function (oscillator/gain/filter setup for the delete "whoosh" and keep "chime" sounds), untestable in jsdom and creating a brand-new AudioContext on every call.
• Move it to src/sound.ts, unchanged in what it plays:
  • playActionSound(action): void — no enabled parameter; FileViewer keeps the settings.soundEffects check itself, since it owns "when to play"
  • a lazily-created, module-level singleton AudioContext, reused across calls instead of created fresh each time
  • if construction fails once, that's cached so later calls skip retrying and re-logging (an environment-support gap, not transient) — scoped only to construction, not to a synthesis-time failure
• Promotes the "delete" | "keep" union (independently duplicated across sound.ts, types.ts, FileViewer.tsx, and App.tsx) to one exported FileAction type in types.ts, per code review.

Test plan

• npm run typecheck passes (all 3 configs)
• npm test — 54/54 pass (4 new: sound.test.ts covers constructing on delete/keep, reusing the same context across calls, and not throwing when AudioContext is unavailable — no assertions on exact frequencies/envelopes, since those are aesthetic, not correctness-critical)
• npm run build — Vite + Electron compile cleanly
• npx biome check clean on changed files
• Reviewed against CONTRIBUTING.md standards and the original design (two-axis review); Spec review clean (numeric values verified byte-for-byte identical to the original), one Standards finding (duplicated "delete" | "keep" literal) fixed pre-commit

🤖 Generated with Claude Code