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/snippets/SnippetsSidebar.tsx

@@ -63,7 +63,7 @@ export const SnippetsSidebar: React.FC<SnippetsSidebarProps> = ({ onItemSelect }
         <h2 className="text-base font-semibold text-foreground mb-3">{t('settings.snippets.sidebar.title')}</h2>
         <div className="flex items-center justify-between gap-2">
           <span className="typography-meta text-muted-foreground">{t('settings.snippets.sidebar.total', { count: snippets.length })}</span>
-          <Button size="sm" variant="ghost" className="h-7 w-7 px-0 -my-1 text-muted-foreground" onClick={handleCreateNew} aria-label={t('settings.snippets.sidebar.actions.create')}>
+          <Button size="sm" data-settings-item="snippets.create" variant="ghost" className="h-7 w-7 px-0 -my-1 text-muted-foreground" onClick={handleCreateNew} aria-label={t('settings.snippets.sidebar.actions.create')}>
             <Icon name="add" className="h-3.5 w-3.5" />
           </Button>
         </div>