This commit refactors MemoView and MemoEditor components for better
maintainability, introducing React Context, custom hooks, and improved
folder structure.

MemoView improvements:
- Introduce MemoViewContext to eliminate prop drilling
- Reduce MemoHeader props from 18 to 8
- Reduce MemoBody props from 9 to 4
- Extract custom hooks: useMemoViewDerivedState, useMemoEditor,
  useMemoHandlers for better separation of concerns
- Fix React hooks ordering bug in edit mode

MemoEditor improvements:
- Extract state management into useMemoEditorState hook
- Extract keyboard handling into useMemoEditorKeyboard hook
- Extract event handlers into useMemoEditorHandlers hook
- Extract initialization logic into useMemoEditorInit hook
- Reduce main component from 461 to 317 lines (31% reduction)

Folder structure cleanup:
- Move SortableItem to memo-metadata (correct location)
- Move ErrorBoundary to components folder
- Flatten Toolbar/InsertMenu structure (remove unnecessary nesting)
- Consolidate hooks in main hooks folder
- Consolidate types in main types folder

Benefits:
- Better separation of concerns
- Improved testability
- Easier maintenance
- Cleaner code organization
- No functionality changes

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>