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/lib/nav.ts

@@ -1,6 +1,7 @@
 import {
   BarChart3,
   type LucideIcon,
+  NotebookPen,
   Plus,
   Settings,
   Users,
@@ -26,5 +27,6 @@ export const navItems: NavItem[] = [
     icon: BarChart3,
     link: "/analysis",
   },
+  { id: "notes", labelKey: "nav.notes", icon: NotebookPen, link: "/notes" },
   { id: "settings", labelKey: "nav.settings", icon: Settings, link: "/settings" },
 ];