feat: Sound Design Template Library — genre-specific preset collections and starting configurations

[ChuxiJ]

Summary

Provide genre-specific sound design starting points that configure multiple tracks with appropriate instruments, effects, and AI generation presets. Instead of starting from a blank canvas, users can load a template like "Lo-fi Hip Hop" or "Synthwave" that pre-configures the entire project's sonic palette.

Acceptance Criteria

• Define SoundDesignTemplate type: { name, genre, description, tracks: TrackTemplate[], generationDefaults }
• TrackTemplate: instrument settings, effect chain preset, AI stem description, suggested role
• Ship 10+ genre templates:
  • Lo-fi Hip Hop (mellow keys, vinyl drums, warm bass, ambient textures)
  • Synthwave/Retrowave (analog polysynth, arpeggiated bass, 80s drums, pad)
  • Orchestral Cinematic (strings, brass, woodwinds, timpani, choir)
  • Trap/Hip Hop (808 bass, hi-hats, snare, vocal chops, synth lead)
  • Indie Rock (electric guitar, bass guitar, acoustic drums, vocal)
  • EDM/House (supersaw lead, sub bass, 4-on-floor kick, pluck)
  • Jazz (piano, upright bass, brush drums, horn section)
  • Ambient/Soundscape (granular pad, reverb-heavy keys, field recordings)
  • Pop (modern drums, synth bass, vocal, piano/keys)
  • Classical Chamber (piano, violin, cello, clarinet)
• Template browser in "New Project" dialog and as a menu option
• Template preview: short audio demo per template
• Users can save current project setup as a custom template
• Templates include AI generation prompt suggestions per track

Technical Notes

• Templates stored as bundled JSON in src/data/templates/
• Each template creates a project with pre-configured tracks (empty of audio but with instrument/effect settings)
• Integrates with Preset Management (feat: Preset Management System — save, load, browse, and categorize instrument presets #1230) for individual instrument presets
• Integrates with AI Timbre Presets (feat: AI Timbre Presets for Generation — curated timbre profiles with reference audio influence #1235) for generation prompt templates

Part of #1229