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/globals.css

@@ -140,3 +140,142 @@ body {
   display: flex;
   overflow: hidden;
 }
+
+/* Milkdown editor overrides */
+[data-milkdown-root] {
+  display: flex;
+  flex-direction: column;
+  height: 100%;
+}
+
+.milkdown {
+  display: flex;
+  flex-direction: column;
+  flex: 1;
+  min-height: 0;
+  color: var(--foreground);
+  font-family: inherit;
+  line-height: 1.75;
+}
+
+.milkdown .editor {
+  flex: 1;
+  outline: none;
+  padding: 0;
+  overflow-y: auto;
+  word-break: break-word;
+  overflow-wrap: break-word;
+}
+
+.milkdown .editor > * + * {
+  margin-top: 0.75em;
+}
+
+.milkdown .editor h1 {
+  font-size: 1.875rem;
+  font-weight: 700;
+  line-height: 1.2;
+  color: var(--foreground);
+}
+
+.milkdown .editor h2 {
+  font-size: 1.5rem;
+  font-weight: 600;
+  line-height: 1.3;
+  color: var(--foreground);
+}
+
+.milkdown .editor h3 {
+  font-size: 1.25rem;
+  font-weight: 600;
+  line-height: 1.4;
+  color: var(--foreground);
+}
+
+.milkdown .editor h4 {
+  font-size: 1.125rem;
+  font-weight: 600;
+  color: var(--foreground);
+}
+
+.milkdown .editor h5,
+.milkdown .editor h6 {
+  font-size: 1rem;
+  font-weight: 600;
+  color: var(--foreground);
+}
+
+.milkdown .editor p {
+  color: var(--foreground);
+}
+
+.milkdown .editor blockquote {
+  border-left: 3px solid var(--border);
+  padding-left: 1rem;
+  color: var(--muted-foreground);
+  font-style: italic;
+}
+
+.milkdown .editor pre {
+  background: var(--muted);
+  color: var(--foreground);
+  border: 1px solid var(--border);
+  border-radius: var(--radius);
+  padding: 0.75rem 1rem;
+  overflow-x: auto;
+  font-family: ui-monospace, SFMono-Regular, 'SF Mono', Menlo, Consolas, monospace;
+  font-size: 0.875rem;
+}
+
+.milkdown .editor code {
+  background: var(--muted);
+  color: var(--foreground);
+  padding: 0.125rem 0.375rem;
+  border-radius: calc(var(--radius) - 4px);
+  font-family: ui-monospace, SFMono-Regular, 'SF Mono', Menlo, Consolas, monospace;
+  font-size: 0.875em;
+}
+
+.milkdown .editor pre code {
+  background: transparent;
+  padding: 0;
+  border-radius: 0;
+}
+
+.milkdown .editor ul,
+.milkdown .editor ol {
+  padding-left: 1.5rem;
+  color: var(--foreground);
+}
+
+.milkdown .editor ul {
+  list-style-type: disc;
+}
+
+.milkdown .editor ol {
+  list-style-type: decimal;
+}
+
+.milkdown .editor li + li {
+  margin-top: 0.25em;
+}
+
+.milkdown .editor a {
+  color: var(--primary);
+  text-decoration: underline;
+  text-underline-offset: 2px;
+}
+
+.milkdown .editor hr {
+  border: none;
+  border-top: 1px solid var(--border);
+  margin: 1.5em 0;
+}
+
+.milkdown .editor strong {
+  font-weight: 600;
+}
+
+.milkdown .editor em {
+  font-style: italic;
+}