Add Notes page: Tiptap rich-text editor backed by the notes API

- lib/notes.ts: list/get/create/update/delete via the API client.
- New /notes route + nav item (and shared lib/nav.ts entry, i18n key) so it
  shows in the sidebar and command palette.
- components/notes/notes-view.tsx: a note list + "New note", with save/delete
  and toasts.
- components/notes/notes-editor.tsx: a Word-like COSS Toolbar driving Tiptap
  (StarterKit + Placeholder) — bold/italic/underline, H1–H2, bullet/numbered
  lists, undo/redo — plus a title field and Save. Installed @coss/toolbar and
  @coss/toggle-group and the Tiptap packages.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -16,6 +16,11 @@
     "@streamdown/code": "^1.1.1",
     "@streamdown/math": "^1.0.2",
     "@streamdown/mermaid": "^1.0.2",
+    "@tiptap/extension-placeholder": "^3.25.0",
+    "@tiptap/extension-underline": "^3.25.0",
+    "@tiptap/pm": "^3.25.0",
+    "@tiptap/react": "^3.25.0",
+    "@tiptap/starter-kit": "^3.25.0",
     "@xyflow/react": "^12.10.2",
     "ai": "^6.0.193",
     "ansi-to-react": "^6.2.6",