feat: restore scroll position to last viewed block on note switch and app startup

[j4rviscmd]

Summary

• Persist the currently visible BlockNote block ID per note in localStorage
• Restore scroll position synchronously inside the onContentLoaded microtask so the browser never paints the intermediate top-of-document state
• Save position on debounced scroll (500ms), note switch (imperative, before DOM update), and visibilitychange

Test plan

• Open a note, scroll down, switch to another note, switch back → position restored
• Open a note, scroll down, close and reopen the app → position restored
• Delete the block that was saved as scroll anchor → falls back to top
• Create a new note (no saved position) → starts at top as expected

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