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/style.css

@@ -23,4 +23,12 @@ body {
     @apply rounded-md p-1.5 text-neutral-500 transition hover:bg-black/5 focus:outline-none
       focus-visible:ring-2 focus-visible:ring-brand dark:text-neutral-400 dark:hover:bg-white/10;
   }
+  .nav-link {
+    @apply flex items-center gap-2 rounded-lg px-3 py-2 font-medium text-neutral-600 transition
+      hover:bg-neutral-200/60 focus:outline-none focus-visible:ring-2 focus-visible:ring-brand
+      dark:text-neutral-300 dark:hover:bg-neutral-800;
+  }
+  .nav-link-active {
+    @apply bg-neutral-200 text-neutral-900 dark:bg-neutral-800 dark:text-neutral-100;
+  }
 }