feat: restore cursor position when reopening notes

[j4rviscmd]

Summary

• Persist the focused BlockNote block ID per note in editorStateStore (editor-state.json) and restore the cursor to that block's end when a note's content finishes loading
• Falls back to the first block's end when no saved position exists
• Handles app startup focus by observing splash screen DOM removal via MutationObserver

Changes

• New: src/shared/lib/cursorPosition.ts — Pure functions for cursor position operations (getFocusedBlockId, placeCursorAtBlockEnd, placeCursorAtFirstBlock)
• New: src/shared/hooks/useCursorMemory.ts — Hook that persists/restores cursor positions per note using tauri-plugin-store, mirrors the existing useBlockScrollMemory pattern
• Modified: src/App.tsx — Integrated useCursorMemory into the editor lifecycle; cursor position is saved on note switch and visibility change, restored via onContentLoaded callback

Test plan

• Switch between multiple notes and verify cursor restores to the last-edited block's end
• Restart the app and verify cursor restores to the previously focused block
• Create a new note and verify cursor falls back to the first block's end
• Delete a block that had a saved cursor position and verify fallback to first block
• Verify splash screen dismissal does not steal focus from the editor on startup

🤖 Generated with Claude Code