feat: add dark mode with persistent user preference

Add dark mode support using Mantine's built-in color scheme system,
persisted via a new Zustand preferences store. Includes a quick toggle
in the app header and an enabled switch in User Preferences. Also
removes the "AI Health Scores" title from the dashboard to reclaim
vertical space.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/main.tsx

@@ -10,6 +10,7 @@ import '@mantine/dates/styles.css';
 import '@mantine/notifications/styles.css';
 import { App } from './App';
 import { theme } from './theme/theme';
+import { usePreferencesStore } from './stores/preferencesStore';
 
 const queryClient = new QueryClient({
   defaultOptions: {
@@ -21,9 +22,11 @@ const queryClient = new QueryClient({
   },
 });
 
-ReactDOM.createRoot(document.getElementById('root')!).render(
-  <React.StrictMode>
-    <MantineProvider theme={theme}>
+function Root() {
+  const colorScheme = usePreferencesStore((s) => s.colorScheme);
+
+  return (
+    <MantineProvider theme={theme} forceColorScheme={colorScheme}>
       <Notifications position="top-right" />
       <ModalsProvider>
         <QueryClientProvider client={queryClient}>
@@ -33,5 +36,11 @@ ReactDOM.createRoot(document.getElementById('root')!).render(
         </QueryClientProvider>
       </ModalsProvider>
     </MantineProvider>
+  );
+}
+
+ReactDOM.createRoot(document.getElementById('root')!).render(
+  <React.StrictMode>
+    <Root />
   </React.StrictMode>,
 );