feat(desktop): add shared output routing and Web adapters

[grazzolini]

Summary

• Add installation-local output routing preferences for project audio, stable stems, and cues. Refined drum parts inherit the Drums route unless overridden. Selections distinguish Inherit, System Default, and an explicit device.
• Keep native and browser device identities separate; Android explicit IDs last only for the process. Preserve saved preferences while exposing effective destinations and fallback state.
• Connect the shared model to native routing and capability-gated Web sinks. Followed metronome clicks stop with project playback; independent Tools playback stays available.
• If selected and System Default outputs both fail, playback stops while retaining position and route preferences.

Motivation

Refs #505. Layer 2 of a three-PR review stack, based on #574. The complete stack is the runnable acceptance target; Playback and Project Studio controls, docs, and release media follow in layer 3. Device-clock synchronization, drift and latency correction, and per-output volume controls remain deferred.

Testing

• Focused routing and metronome suites: 99 passed:
  pnpm --filter @tuneforge/desktop exec vitest run src/lib/audioOutput.routing.test.ts src/lib/outputRouting.test.ts src/lib/AudioOutputProvider.test.tsx src/features/projects/playback.race-regressions.test.tsx src/App.tools-metronome.test.tsx src/App.tools-chord-dictionary-responsive.test.tsx
• Project playback integration suites: 105 passed:
  pnpm --filter @tuneforge/desktop exec vitest run src/features/projects/playback.test.tsx src/App.project-precount.test.tsx src/App.project-media-controls.test.tsx src/App.project-playback-stems.test.tsx
• pnpm typecheck, scoped ESLint on changed desktop files, and git diff --check passed.
• Complete-stack validation follows after all three PRs are open: pnpm lint, pnpm typecheck, pnpm test, native/platform checks, Android and Linux builds, hardware routing, and node scripts/capture-release-media.mjs --run.

Checklist

• Conventional title; focused tests and review complete
• No secrets or copyrighted audio added
• Complete-stack integration and platform validation
• UI, docs, changelog, and release media in layer 3

The merge-base changed after approval.