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/app/(app)/notes/page.tsx

@@ -0,0 +1,10 @@
+import { NotesView } from "@/components/notes/notes-view";
+import { SidebarInset } from "@/components/ui/sidebar";
+
+export default function NotesPage() {
+  return (
+    <SidebarInset className="flex flex-1 flex-col overflow-hidden">
+      <NotesView />
+    </SidebarInset>
+  );
+}