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}

frontend/src/pages/preferences/UserPreferencesPage.tsx

@@ -6,9 +6,11 @@ import {
   IconUser, IconPalette, IconClock, IconBell, IconEye,
 } from '@tabler/icons-react';
 import { useAuthStore } from '../../stores/authStore';
+import { usePreferencesStore } from '../../stores/preferencesStore';
 
 export function UserPreferencesPage() {
   const { user, currentOrg } = useAuthStore();
+  const { colorScheme, toggleColorScheme } = usePreferencesStore();
 
   return (
     <Stack>
@@ -66,7 +68,10 @@ export function UserPreferencesPage() {
                 <Text size="sm">Dark Mode</Text>
                 <Text size="xs" c="dimmed">Switch to dark color theme</Text>
               </div>
-              <Switch disabled />
+              <Switch
+                checked={colorScheme === 'dark'}
+                onChange={toggleColorScheme}
+              />
             </Group>
             <Group justify="space-between">
               <div>
@@ -76,7 +81,7 @@ export function UserPreferencesPage() {
               <Switch disabled />
             </Group>
             <Divider />
-            <Text size="xs" c="dimmed" ta="center">Display preferences coming in a future release</Text>
+            <Text size="xs" c="dimmed" ta="center">More display preferences coming in a future release</Text>
           </Stack>
         </Card>