feat(cli): enhance TUI editor with editing, navigation, and UI polish

Core editor improvements:
- Cursor navigation in edit mode (Left/Right/Home/End, Ctrl+A/E)
- Backspace and forward-delete (Delete key) with char-safe UTF-8 handling
- Ctrl+Y copies source-language translation into edit buffer
- One-level undo (u) for the last committed translation edit
- Delete key with confirmation dialog (D → y/n)
- Missing-translation jump navigation (] / [)
- Filter mode cycle (F) to show only keys with missing translations
- Horizontal panel resize (< / >) with split_ratio 20–70%
- Scrollable translations panel (Alt+Down/Up or Shift+J/K)
- Page navigation (Ctrl+D/U)
- g/G jump to top/bottom of key list

UI redesign:
- Rounded borders on all panels
- Yellow ! prefix on keys with missing translations
- Cursor rendered with REVERSED style in edit mode
- Position indicator (pos/total) in key panel title
- Help overlay (? key) with all keybindings
- Status bar with distinct colors per input mode
- ConfirmDelete and ConfirmQuit modal status hints

Bug fixes:
- Force terminal.clear() on key navigation to eliminate complex-script
  (Bengali/Hindi/Arabic) glyph artifacts from ratatui's diff renderer
- Per-row Rect rendering (height:1) for non-selected translations
  prevents line-wrap from misaligning the translation list

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: WendellXY