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/components/sections/usage/UsagePage.tsx

@@ -170,7 +170,7 @@ export const UsagePage: React.FC = () => {
         </div>
 
         {/* Options */}
-        <div className="mb-8 px-2">
+        <div data-settings-item="usage.header-menu" className="mb-8 px-2">
           <div
             className="group flex cursor-pointer items-center gap-2 py-1.5"
             role="button"
@@ -228,7 +228,7 @@ export const UsagePage: React.FC = () => {
 
         {/* Overall Usage Windows */}
         {usage?.windows && Object.keys(usage.windows).length > 0 && (
-          <div className="mb-8">
+          <div data-settings-item="usage.model-quotas" className="mb-8">
             <section className="px-2 pb-2 pt-0">
               <div className="divide-y divide-[var(--surface-subtle)]">
                 {Object.entries(usage.windows).map(([label, window]) => (