feat: add pan, zoom, and auto-fit to whiteboard canvas

[YizukiAme]

Summary

Fixes #19 — Whiteboard content that exceeds the fixed canvas bounds was clipped by overflow-hidden with no way to view hidden parts.

Changes

components/whiteboard/whiteboard-canvas.tsx

• Auto-fit scaling: When elements overflow the 1000×562.5 canvas, computes bounding box and applies scale + translate to ensure all content is visible (with 24px padding). When content fits normally, no transform is applied — zero visual regression.
• Drag-to-pan: Hold mouse and drag to pan around the whiteboard (pointer events with capture)
• Scroll-to-zoom: Mouse wheel zooms in/out (0.2x–5x range), using native addEventListener({ passive: false }) for proper preventDefault
• Double-click reset: Double-click or click "Reset View" button to return to default view (with smooth 0.25s animation)
• Text selection prevention: select-none + preventDefault prevents accidental text selection during drag/double-click
• Auto-reset on content change: View resets when whiteboard content changes (tracked by stable element ID key, not object reference)
• Grab cursor: Shows grab/grabbing cursor when pan/zoom is available

lib/i18n/stage.ts

• Added resetView translation key for zh-CN ("重置视图") and en-US ("Reset View")

How it works

Content fits canvas → No change, original behavior
Content overflows → Auto-fit scales down to show all
User scrolls wheel → Zoom in/out (centered)
User drags → Pan around
User double-clicks/button → Smooth reset to auto-fit
New content loads → Auto-reset view

Testing

Tested with injected overflow elements (left/right/bottom + negative coordinates) and confirmed:

• ✅ Auto-fit scales content to fit
• ✅ Scroll zoom works
• ✅ Drag-to-pan works (no text selection)
• ✅ Double-click reset with smooth animation
• ✅ Reset View button appears and works
• ✅ No visual regression when content fits normally