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/router/index.ts

@@ -23,6 +23,12 @@ const router = createRouter({
       component: () => import("../views/BoardView.vue"),
       meta: { requiresAuth: true },
     },
+    {
+      path: "/label/:id",
+      name: "label",
+      component: () => import("../views/BoardView.vue"),
+      meta: { requiresAuth: true },
+    },
     {
       path: "/settings",
       name: "settings",