fix: restore per-note header visibility state on note switch

[j4rviscmd]

Summary

• Refactored headerHidden from a single global boolean to a per-note Record<string, boolean> map (headerHiddenMap), matching the pattern used by scroll/cursor position memory
• Added noteId parameter to useScrollDirection hook with automatic state restoration via useEffect
• Removed imperative resetHeader() call from selectNote() in favor of reactive noteId-based restoration
• Added one-time migration from legacy headerHidden boolean key to the new map format
• Added mapLoadedRef guard to prevent race condition between async store load and noteId change effect
• Added scroll-to-top fallback in useBlockScrollMemory when no saved scroll position exists for a note, fixing content appearing cut off when opening new notes from a hidden-header note

Test plan

• Open note A, scroll down to hide header, switch to note B → header should be visible (B has no saved state)
• Switch back to note A → header should be restored to hidden state
• Create a new note from a note with hidden header → new note should open scrolled to top with header visible
• Restart app → last opened note's header state should be restored correctly
• Verify legacy headerHidden boolean is migrated and cleaned up on first launch

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