Lands #4175 (token-driven generative ambient music in the desktop app) on current main-v2.

Squashed onto main-v2 (the original PR had merged main-v2 back in and #4188 had since moved SettingsPanel.tsx), plus a house-style comment pass on generative-music.ts / App.tsx — trimmed the multi-line file header, dropped the // ── … ── section banners and the restating one-liners, keeping only the non-obvious why (pentatonic scale, the double throttle, the wet gain). No behaviour change: the code lines are byte-identical to #4175.

Credit to @ttmouse for the feature and @SivanCola for the review fixes (default-off + live preset/stop sync), both folded into the single commit here.

What it does

• Opt-in, default off. Preset picker in Settings → General (classic / ethereal / digital / retro).
• Each text/reasoning/tool token triggers a C-major-pentatonic note through a Web Audio chain (oscillator → ADSR → low-pass → reverb); fast generation → denser notes.
• Throttled (every 4th token + 100 ms floor) for DeepSeek's output rate.
• Switching to Off or another preset mid-generation takes effect immediately; nothing auto-plays on launch/upgrade.

Verification (local)

• pnpm --dir desktop/frontend typecheck — clean apart from the usual ungenerated-wailsjs binding errors that wails generate resolves in CI.
• go build ./... in desktop/ — green.
• Confirmed the cleanup is comment-only (PR-original vs. cleaned: code lines identical).

Closes #4175