BREAKING CHANGE: MemoEditor internal architecture completely refactored

## Summary

Refactored MemoEditor from hooks-based state management to a three-layer
architecture (Presentation → State → Services) using useReducer pattern.

## Changes

### Architecture
- **State Layer** (5 new files): types, actions, reducer, context, barrel export
- **Service Layer** (6 new files): error, validation, upload, cache, memo services + barrel
- **Component Layer** (3 new files): EditorToolbar, EditorContent, EditorMetadata
- **Simplified Hooks** (3 new files): useMemoInit, useAutoSave, useKeyboard

### Code Reduction
- Main component: ~380 lines → ~140 lines (-63%)
- Hooks removed: 5 old hooks (useMemoEditorState, useMemoSave, etc.)
- Total lines removed: 508 lines of old code
- Utility hooks preserved: 8 hooks still in use (useLocation, useDragAndDrop, etc.)

### Improvements
- ✅ Predictable state transitions with useReducer
- ✅ Testable business logic in pure service functions
- ✅ Cleaner component code (presentation only)
- ✅ Better separation of concerns
- ✅ Type-safe actions with discriminated unions
- ✅ Centralized error handling

## Statistics
- Files changed: 26
- Commits created: 25 (squashed into 1)
- New files: 17
- Removed files: 5
- TypeScript errors: 0
- Lint errors: 0

## Testing
Manual testing required for:
- Editor functionality (create, edit, save)
- Drag and drop
- Focus mode
- Keyboard shortcuts (Cmd/Ctrl + Enter)
- Auto-save to localStorage