Consolidate tab persistence logic into Tabs component

- Move active tab persistence into Tabs component with storageKey + activeTabKey props
- Change value prop to defaultValue so callers don't manage tab state
- Add TabsRef with setActiveTab method for programmatic tab switching
- Restore request_pane.focus_tab listener for :param placeholder clicks
- Update all Tab consumers to use new pattern

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src-web/components/Settings/SettingsPlugins.tsx

@@ -54,13 +54,11 @@ export function SettingsPlugins({ defaultSubtab }: SettingsPluginsProps) {
   const installedPlugins = plugins.filter((p) => !isPluginBundled(p, appInfo.vendoredPluginDir));
   const createPlugin = useInstallPlugin();
   const refreshPlugins = useRefreshPlugins();
-  const [tab, setTab] = useState<string | undefined>(defaultSubtab);
   return (
     <div className="h-full">
       <Tabs
-        value={tab}
+        defaultValue={defaultSubtab}
         label="Plugins"
-        onChangeValue={setTab}
         addBorders
         tabs={[
           { label: 'Discover', value: 'search' },