feat(agent): model quick-picker in overflow menu

Adds three one-click model shortcuts (haiku / sonnet / opus) at the
bottom of the agent overflow menu (⋯), below a separator and a small
'model' section label.

- The active model alias is highlighted with .active (purple border +
  tint) so the operator can see at a glance which model is current.
  Matching tolerates both short aliases (haiku) and full API names
  (claude-3-5-haiku-20241022) via an endsWith check.
- Clicking the active model is a no-op (closes menu, no POST).
- Clicking a different alias calls postModel() and closes the menu.
- renderModelChip() keeps the picker buttons in sync on every
  /api/state refresh without rebuilding the menu.

Before this change the operator had to type /model <name> in the
compose box. The overflow menu provides a discoverability path for
operators who don't remember slash commands.

frontend/packages/agent/src/agent.css

@@ -263,6 +263,32 @@ h2, h3 {
   opacity: 0.4;
   cursor: progress;
 }
+.overflow-sep {
+  height: 1px;
+  background: var(--border);
+  margin: 0.3em 0.4em;
+}
+.overflow-section-label {
+  font-size: 0.72em;
+  letter-spacing: 0.1em;
+  color: var(--muted);
+  padding: 0 0.7em 0.1em;
+  text-transform: uppercase;
+}
+.overflow-item-model {
+  color: var(--muted);
+  font-variant-numeric: tabular-nums;
+}
+.overflow-item-model:hover {
+  color: var(--fg);
+  background: rgba(203, 166, 247, 0.08);
+  border-color: var(--purple-dim);
+}
+.overflow-item-model.active {
+  color: var(--purple);
+  border-color: var(--purple-dim);
+  background: rgba(203, 166, 247, 0.06);
+}
 
 /* Header pill — inbox / loose-ends triggers. Compact, count-prominent. */
 .header-pill {