fix: prevent header from scrolling away by constraining layout height

[j4rviscmd]

Summary

• Fix header scrolling out of view by adding h-svh to SidebarProvider, establishing a proper height chain so SidebarInset becomes the scroll container
• Move scroll responsibility from Editor to a wrapper div, and replace Editor's <main> with <div> to resolve semantic HTML nesting
• Simplify note selection by consolidating localStorage sync into a single selectNote callback

Test plan

• Verify header stays fixed while scrolling through a long note
• Verify header remains visible with an empty/new note
• Verify sidebar open/close doesn't affect header positioning
• Verify dark mode header background renders correctly
• Verify BlockNote toolbar and editor scrolling work without interference

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