Add undo/redo to planner

Yjs UndoManager tracks waypoints, no-go areas, and notes. Only
local mutations (origin "local") are undoable — remote changes
from other users are not.

- Keyboard shortcuts: Ctrl/Cmd+Z (undo), Ctrl/Cmd+Shift+Z/Y (redo)
- Shortcuts suppressed in text inputs (browser-native undo there)
- Undo/redo buttons in header with disabled state
- stopCapturing on drag to isolate drag as one undo step
- All mutation sites wrapped with "local" origin
- 5 unit tests for UndoManager behavior
- Archived undo-redo change

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/planner/app/components/NotesPanel.tsx

@@ -42,7 +42,7 @@ export function NotesPanel({ yjs }: NotesPanelProps) {
       yjs.doc.transact(() => {
         yjs.notes.delete(0, yjs.notes.length);
         yjs.notes.insert(0, newValue);
-      });
+      }, "local");
       isLocalChange.current = false;
     },
     [yjs.notes, yjs.doc],