M1: masonry board UI — quick-add, note cards, editor, views

- notes Pinia store (load/create/pin/archive/color/trash/restore/delete) with
  view-aware reconcile; api client patch/del methods.
- colors.ts palette (10 keys → light/dark card + swatch tints).
- QuickAdd (collapsed → expand, title/body/color, click-outside/Esc to save),
  NoteCard (color tint, click-to-edit, hover action bar), NoteEditor modal,
  ColorPicker, inline Icon set (no icon dep).
- BoardView rewrite: Notes/Archive/Trash routes, CSS-columns masonry, Pinned +
  Others sections, per-view empty states, sign-out.

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

frontend/src/style.css

@@ -17,3 +17,10 @@ body {
     @apply bg-neutral-950 text-neutral-100;
   }
 }
+
+@layer components {
+  .icon-btn {
+    @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;
+  }
+}