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.

progress.txt

@@ -269,3 +269,29 @@
   - `notes.create` returns `{ id }` which can be used directly for navigation in the `onSuccess` callback
   - TanStack Router file-based routing: `notes.$noteId.tsx` generates `/notes/:noteId` route automatically — `Route.useParams()` provides typed `{ noteId }`
 ---
+
+## 2026-02-22 - US-016
+- What was implemented:
+  - Installed `@milkdown/kit`, `@milkdown/react`, `@milkdown/theme-nord` (following official Milkdown installation guide)
+  - Created `MilkdownEditor` wrapper component at `src/renderer/components/notes/MilkdownEditor.tsx`
+  - Uses official React recipe: `MilkdownProvider` + `useEditor` hook with `Editor.make()` configuring `commonmark`, `listener`, `history` plugins
+  - `listenerCtx.markdownUpdated()` fires onChange callback via stable `useRef` (avoids editor re-creation)
+  - `defaultValueCtx` sets initial markdown content from SQLite
+  - Rewrote `src/renderer/routes/notes.$noteId.tsx` with full editor page:
+    - Editable title: borderless shadcn/ui `Input` (border-0, shadow-none, focus-visible:ring-0), saves on blur via `notes.update({ id, title })`
+    - Auto-save: `onChange` from Milkdown triggers 500ms debounced `notes.update({ id, content })` via `useRef` + `setTimeout`/`clearTimeout`
+    - "Saving..." indicator: shadcn/ui `Badge` (variant=secondary) shown while debounce is pending, hidden on mutation `onSettled`
+    - Back button: shadcn/ui `Button` (variant=ghost, size=icon) with `ArrowLeft` Lucide icon, `window.history.back()`
+    - Loading/not-found states handled
+  - Added Milkdown/ProseMirror CSS overrides in `src/renderer/globals.css` using semantic color variables (`var(--foreground)`, `var(--muted)`, `var(--border)`, `var(--muted-foreground)`, `var(--primary)`)
+  - Typecheck passes (zero errors)
+- Files changed: `src/renderer/components/notes/MilkdownEditor.tsx` (new), `src/renderer/routes/notes.$noteId.tsx`, `src/renderer/globals.css`, `package.json`, `package-lock.json`, `prd.json`, `progress.txt`
+- **Learnings for future iterations:**
+  - `@milkdown/kit` is the recommended all-in-one package — it bundles core, preset-commonmark, plugin-listener, plugin-history, and utilities under sub-paths like `@milkdown/kit/core`, `@milkdown/kit/preset/commonmark`, etc.
+  - The `useEditor` hook from `@milkdown/react` takes `(root) => Editor.make()...` — the `root` param is the DOM element Milkdown manages, set via `ctx.set(rootCtx, root)`
+  - Use `useRef` for the onChange callback passed to `listenerCtx.markdownUpdated()` — this avoids re-creating the editor instance when the callback identity changes
+  - `listenerCtx.markdownUpdated((_ctx, markdown, prevMarkdown))` provides both current and previous markdown — compare them to avoid firing on no-op updates
+  - For debounced auto-save: `useRef<ReturnType<typeof setTimeout>>` + `clearTimeout`/`setTimeout` is simpler than external debounce libraries; cleanup in `useEffect` return prevents stale saves
+  - Nord theme (`@milkdown/theme-nord`) provides base ProseMirror structure; override with CSS using the app's semantic color variables for consistent theming
+  - Import both `@milkdown/theme-nord/style.css` and `@milkdown/kit/prose/view/style/prosemirror.css` for proper base styling
+---