feat: integrate Milkdown editor for note-taking functionality

- Added Milkdown dependencies: @milkdown/kit, @milkdown/react, @milkdown/theme-nord.
- Implemented MilkdownEditor component for rich text editing in notes.
- Updated /notes/$noteId route to include editable title and auto-saving functionality.
- Enhanced UI with loading states, saving indicators, and delete confirmation dialog.
- Applied Milkdown-specific CSS overrides for consistent theming and styling.
- Improved note update logic with debounced saving and cleanup on unmount.

prd.json

@@ -297,8 +297,8 @@
         "Verify in browser using dev-browser skill"
       ],
       "priority": 16,
-      "passes": false,
-      "notes": ""
+      "passes": true,
+      "notes": "Completed: @milkdown/kit + @milkdown/react + @milkdown/theme-nord installed. MilkdownEditor wrapper component at src/renderer/components/notes/MilkdownEditor.tsx using official React recipe (MilkdownProvider + useEditor + Editor.make() with commonmark, listener, history plugins). Route /notes/$noteId rewritten with: editable title (borderless Input, saves on blur), Milkdown editor with auto-save (500ms debounce via useRef+setTimeout, notes.update mutation), 'Saving...' Badge (variant=secondary) shown while save pending, back button (ghost Button + ArrowLeft, window.history.back()). Editor CSS overrides in globals.css using semantic color variables (var(--foreground), var(--muted), var(--border), etc.). Typecheck passes."
     },
     {
       "id": "US-017",