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/behavior/BehaviorPage.tsx

@@ -243,7 +243,7 @@ export const BehaviorPage: React.FC = () => {
           </h2>
         </div>
 
-        <div>
+        <div data-settings-item="behavior.system-prompt">
           <div className="mb-1 px-1">
             <div className="flex items-center gap-1.5">
               <h3 className="typography-ui-header font-medium text-foreground">
@@ -288,7 +288,7 @@ export const BehaviorPage: React.FC = () => {
           </section>
         </div>
 
-        <div>
+        <div data-settings-item="behavior.response-style">
           <div className="mb-1 px-1">
             <div className="flex items-center gap-1.5">
               <h3 className="typography-ui-header font-medium text-foreground">