feat(settings): add item search (#1592)

Adds item-level search inside Settings so users can find concrete settings like provider auth, agent mode, terminal font size, tunnel options, notification events, and similar controls instead of only filtering top-level pages.
Groups search results by Settings page and shows localized labels plus optional descriptions where useful.
Supports keyboard navigation with Arrow Up/Down, Enter, and Escape, matching the existing autocomplete interaction style.
Opens the correct Settings page or split-page draft state before scrolling to the matching control.
Highlights the matched setting with a subtle token-based background so users can see where they landed without an aggressive outline.
Adds explicit data-settings-item anchors across Settings pages and a centralized search registry with runtime/mobile availability guards.
Updates Settings UI skill guidance so future Settings changes keep search registry entries, anchors, localization, and availability guards in sync.

packages/ui/src/index.css

@@ -40,6 +40,12 @@ button,
   animation: none !important;
 }
 
+[data-settings-search-highlight="true"] {
+  border-radius: 0.25rem;
+  background: color-mix(in srgb, var(--primary-base) 7%, transparent);
+  transition: background-color 180ms ease;
+}
+
 /* ── macOS vibrancy ───────────────────────────────────────────────────────
    The window is created transparent with a native 'sidebar' NSVisualEffectView
    behind it. We keep the whole app opaque (bg-background covers the window)