Safari trackpad zoom polish and PWA install support

[vgomx]

Summary

• Improve Safari trackpad pinch/pan smoothness by normalizing wheel deltas, clamping large pinch steps, coalescing viewport transforms to one rAF update per frame, and adding GPU compositing hints
• Add installable PWA support via vite-plugin-pwa (manifest + service worker) with an Install section in Settings and Chrome beforeinstallprompt handling

Test plan

• Safari: pinch-zoom and two-finger pan on canvas — should feel smoother and less jumpy than before
• Chrome: verify trackpad zoom/pan still works normally
• Settings → Install: confirm instructions show per platform; Chrome install button appears when eligible
• Chrome: install from browser menu or in-app button; app opens standalone
• Safari macOS: File → Add to Dock works after deploy with HTTPS
• npm test (157 tests) and npm run build

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