feat: add auto-hide header that responds to scroll direction

[j4rviscmd]

Summary

• Add useScrollDirection hook that tracks scroll direction via wheel events (not scroll) to avoid false positives from ProseMirror's scrollIntoView
• Header collapses with max-height: 0 animation when scrolling down and reappears when scrolling up, reclaiming editor space (no visual gap)
• Disable macOS rubber-band bounce at both app level (html, body { overflow: hidden }) and editor container level (overscroll-none)

Details

useScrollDirection hook (src/shared/hooks/useScrollDirection.ts)

• Uses wheel events instead of scroll to ignore programmatic scrolling from BlockNote/ProseMirror
• 200ms click lock prevents macOS synthetic wheel events on mousedown from toggling the header
• Dead zone threshold (10px) prevents jittery toggling from small scroll movements
• Always shows header when scrollTop === 0

Layout changes (src/App.tsx)

• Header collapses via max-height transition (not transform) so the editor reclaims the space
• Scroll container gets overscroll-none to prevent bounce at content edges

Global styles (src/index.css)

• html, body { overflow: hidden } prevents app-level rubber-band bounce on macOS

Test plan

• Scroll down in the editor — header should hide smoothly
• Scroll up — header should reappear
• Click anywhere in the editor — header should NOT toggle (click lock)
• Move cursor with keyboard — header should NOT toggle (no wheel event)
• Scroll to top — header should always be visible
• Scroll to bottom — no bounce, no header flicker
• Toggle sidebar — layout should remain correct

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