feat: infinite canvas with pan, zoom & dot-grid background

[umohsamuel]

Description

Adds Canva/Figma-style infinite canvas navigation to the editor. The artboard is now positioned freely in the viewport and can be panned and zoomed using mouse/trackpad gestures.

Updates

Pan & scroll

1. Scroll wheel pans the canvas vertically (trackpad horizontal scroll also works)
2. Shift + scroll wheel pans horizontally
3. Viewport changed from overflow-auto (native browser scrollbars) to overflow-hidden with manual translate() positioning

Zoom

1. Ctrl/Cmd + scroll wheel zooms toward the cursor position

2. Trackpad pinch-to-zoom is supported

3. Raw delta is clamped to ±25 before converting to a zoom factor, so mouse wheel ticks give ~20% per step while trackpad pinch gives fine-grained fractional zoom

4. Max zoom raised from 100% to 500%

5. Slider zoom and fit-to-view both re-center the artboard by computing matching pan values

Dot-grid background

1. Infinite tiling dot pattern rendered via CSS radial-gradient behind the artboard
2. Dots scale with zoom level (24 × scale px spacing) and track panning via backgroundPosition

Testing

1. All 10 existing tests pass
2. Build completes without errors

[vercel]

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[cubic-dev-ai]

No issues found across 4 files