fix: rAF-coalesce canvas pan for smoothness at low zoom

[trmquang93]

Summary

Fixes backlog 8.3 — canvas pan feels janky at low zoom (~20%) on flows with many screens.

• Coalesce setPan into one call per animation frame for both space-drag pan (mousemove) and trackpad wheel pan. The latest delta wins; intermediate events are dropped, capping update rate to the display refresh rate.
• Mirror panStart into a ref so the rAF callback never reads stale closures across renders.
• Cancel pending rAFs on mouseup and on hook unmount to avoid leaks and "setState on unmounted component" warnings.
• Drag, multi-drag, and pinch-zoom (cmd/ctrl + wheel) branches are untouched. Public hook signature unchanged.

Why this works: at low zoom more screens are visible, so each React render is heavier. Synchronous setPan per mousemove saturates the commit pipeline and frames stutter. rAF coalescing matches update rate to vsync and eliminates the back-pressure.

The pan delta math is in screen-space (CSS transform is translate(pan) scale(zoom), so pan is post-scale); no zoom-divide needed for cursor tracking. See plan §"Math Verification" for detail.

Scope is intentionally minimal (single file, src/hooks/useCanvas.js). Memoization, viewport culling, and connection-geometry decoupling are deferred to a follow-up perf epic.

Test plan

• Smoothness at 20% zoom — open a flow with 20+ screens, hold space and drag in a smooth circle. Motion tracks the cursor with no perceptible stutter.
• No regression at 100% zoom — frame-perfect tracking.
• Mid-pan cursor reversal — direction changes are responsive.
• Pan-tool mode (no space held) — same smoothness.
• Trackpad two-finger pan at 20% zoom — smooth.
• Pinch-zoom (cmd+scroll) — zoom focus point still correct.
• Mid-pan release — no residual canvas drift after mouseup.
• Single-screen drag at 20% zoom — screen still placed exactly under cursor.
• Multi-select drag at 20% zoom — still works.
• Navigate away mid-pan to #landing — no console warnings.
• DevTools Performance — ≤1 React commit per frame during sustained pan.
• npm run lint, npm run build, npm test all pass.