UI polish: notes/tasks list views

- NotesListView: skeleton shimmer for grid and list modes; grid columns change from fixed 3 to auto-fill(260px)
- NoteCard compact: flat border-bottom row style instead of stacked mini-cards; gradient fade mask on grid preview text
- TaskCard compact: flat border-bottom row style; show up to 2 tags
- TasksListView: gradient + shadow on New Task button (matches NotesListView)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/TasksListView.vue

@@ -345,11 +345,16 @@ function toggleGroup(key: string) {
 }
 .btn-new {
   padding: 0.45rem 1rem;
-  background: var(--color-primary);
+  background: linear-gradient(135deg, #6366f1, #4f46e5);
   color: #fff;
   border-radius: var(--radius-sm);
   text-decoration: none;
   font-size: 0.9rem;
+  box-shadow: 0 1px 6px rgba(99, 102, 241, 0.25);
+}
+.btn-new:hover {
+  box-shadow: 0 3px 12px rgba(99, 102, 241, 0.45);
+  filter: brightness(1.08);
 }
 .controls {
   display: flex;