Improve mobile media lightbox gestures

[selfcontained]

Summary

• add a true full-screen image lightbox with contained pinch/wheel zoom and bounded pan
• add 100%-only swipe navigation, double-tap zoom, safe-area-aware controls, and page scroll locking
• split lightbox actions, image controls, text rendering, syntax highlighting, and gesture state into focused modules
• isolate toolbar interactions from canvas double-click/wheel gestures after frontend UX review

Validation

• pnpm run check
• pnpm run finalize:web
• pnpm run test:e2e (168 applicable passed; 12 inert-runtime skips)
• focused Playwright regression for mobile zoom, swipe, and rapid toolbar interactions
• frontend-ux-review persona approved after round 2