M2 labels frontend: sidebar shell + label filter, chips, picker, manage

- AppShell: persistent left sidebar (Notes · labels · Archive · Trash) + top bar
  (site name, admin Settings, sign out); BoardView now renders inside it.
- labels store (list/create/rename/delete); Note gains labels[]; notes store
  gains setLabels + label-aware reconcile + /api/notes?label= loading.
- /label/:id route → label-filtered board.
- LabelPicker (tag a note, create-on-the-fly) in the editor; label chips shown
  on cards and in the editor; LabelsModal to create/rename/delete labels.
- api client PUT; new icons (note/tag/pencil/plus/check); nav-link styles.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/api/client.ts

@@ -30,5 +30,6 @@ export const api = {
   get: <T>(path: string) => request<T>("GET", path),
   post: <T>(path: string, body?: unknown) => request<T>("POST", path, body),
   patch: <T>(path: string, body?: unknown) => request<T>("PATCH", path, body),
+  put: <T>(path: string, body?: unknown) => request<T>("PUT", path, body),
   del: <T>(path: string) => request<T>("DELETE", path),
 };