perf: Tier 1 dashboard rendering optimizations

[aterrylu]

Summary

• CSS spinner replaces setInterval(100ms) + React state → zero JS re-renders during animation
• Polling equality guards on fetchSessions and fetchNotifications skip set() when data unchanged, preventing subscriber re-renders on every 3s/5s poll
• useShallow consolidates 18 separate Zustand subscriptions in Sidebar into 2 focused selectors (data + actions)
• rAF debounce batches notifySlotUpdate from ResizeObserver, coalescing multiple callbacks per frame
• React.memo on GroupContainer prevents unnecessary re-renders from parent state changes

Problem

Dashboard was sluggish when clicking UI elements — React DevTools profiler showed 70+ unnecessary re-renders/sec from:

1. N agents × 10fps JS spinner state updates
2. Polling always writing new objects to store (even when data unchanged)
3. 18 separate Zustand subscriptions in Sidebar (every store change triggers 18 equality checks)
4. Unbatched ResizeObserver notifications causing re-render storms in SessionMountLayer

Changes

Fix	File	Impact
CSS @keyframes spinner	agent-status-icon.tsx	Eliminates N×10fps React renders
fetchNotifications equality guard	store.ts	Skips ~95% of 3s poll writes
useShallow selector split	Sidebar.tsx	18 subscriptions → 2, data vs actions
fetchSessions equality guard	store.ts	Skips ~95% of 5s poll writes
rAF debounce notifySlotUpdate	SessionMountLayer.tsx	Batches resize callbacks per frame
React.memo GroupContainer	Sidebar.tsx	Prevents parent-triggered re-renders

Test plan

• Build passes (npm run build)
• Biome lint clean
• /polish — code-reviewer, code-simplifier, silent-failure-hunter all run
• Manual testing: UI feels noticeably snappier, spinners animate correctly
• Verify agent status transitions still update (working → idle → needs_input)
• Verify session list updates when sessions are created/destroyed
• Verify notification badges update on new messages

🤖 Generated with Claude Code