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.

src/renderer/components/notes/MilkdownEditor.tsx

@@ -0,0 +1,47 @@
+import { useRef } from 'react';
+import { Editor, rootCtx, defaultValueCtx } from '@milkdown/kit/core';
+import { commonmark } from '@milkdown/kit/preset/commonmark';
+import { history } from '@milkdown/kit/plugin/history';
+import { listener, listenerCtx } from '@milkdown/kit/plugin/listener';
+import { Milkdown, MilkdownProvider, useEditor } from '@milkdown/react';
+
+import '@milkdown/kit/prose/view/style/prosemirror.css';
+
+interface MilkdownEditorProps {
+  initialContent: string;
+  onChange: (markdown: string) => void;
+}
+
+function MilkdownInner({ initialContent, onChange }: MilkdownEditorProps) {
+  const onChangeRef = useRef(onChange);
+  onChangeRef.current = onChange;
+
+  useEditor((root) =>
+    Editor.make()
+      .config((ctx) => {
+        ctx.set(rootCtx, root);
+        ctx.set(defaultValueCtx, initialContent);
+      })
+      .use(commonmark)
+      .use(history)
+      .use(listener)
+      .config((ctx) => {
+        ctx.get(listenerCtx).markdownUpdated((_ctx, markdown, prevMarkdown) => {
+          if (markdown !== prevMarkdown) {
+            onChangeRef.current(markdown);
+          }
+        });
+      }),
+    []
+  );
+
+  return <Milkdown />;
+}
+
+export function MilkdownEditor(props: MilkdownEditorProps) {
+  return (
+    <MilkdownProvider>
+      <MilkdownInner {...props} />
+    </MilkdownProvider>
+  );
+}