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/pages/dashboard/DashboardPage.tsx

@@ -414,7 +414,6 @@ export function DashboardPage() {
         <Center h={200}><Loader /></Center>
       ) : (
         <>
-          <Text size="sm" fw={600} c="dimmed">AI Health Scores</Text>
           <SimpleGrid cols={{ base: 1, md: 2 }}>
             <HealthScoreCard
               score={healthScores?.operating || null}