fix: reset header visibility when switching notes

[j4rviscmd]

Summary

• When switching from a scrolled note to a short note, the header remained hidden because the scroll direction state was not reset
• Expose resetHeader from useScrollDirection hook to clear internal state (suppress timer, accumulated wheel delta) and force the header visible
• Call resetHeader() in selectNote when switching notes

Test plan

• Open a long note, scroll down to hide the header
• Switch to a short note (content that doesn't require scrolling)
• Verify the header is visible
• Verify the header still hides/shows correctly when scrolling within a note
• Verify header state is restored correctly on app restart