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/components/layout/AppLayout.tsx

@@ -1,5 +1,5 @@
 import { useState, useEffect } from 'react';
-import { AppShell, Burger, Group, Text, Menu, UnstyledButton, Avatar, Alert, Button } from '@mantine/core';
+import { AppShell, Burger, Group, Text, Menu, UnstyledButton, Avatar, Alert, Button, ActionIcon, Tooltip } from '@mantine/core';
 import { useDisclosure } from '@mantine/hooks';
 import {
   IconLogout,
@@ -9,9 +9,12 @@ import {
   IconUserCog,
   IconUsersGroup,
   IconEyeOff,
+  IconSun,
+  IconMoon,
 } from '@tabler/icons-react';
 import { Outlet, useNavigate, useLocation } from 'react-router-dom';
 import { useAuthStore } from '../../stores/authStore';
+import { usePreferencesStore } from '../../stores/preferencesStore';
 import { Sidebar } from './Sidebar';
 import { AppTour } from '../onboarding/AppTour';
 import { OnboardingWizard } from '../onboarding/OnboardingWizard';
@@ -20,6 +23,7 @@ import logoSrc from '../../assets/logo.svg';
 export function AppLayout() {
   const [opened, { toggle, close }] = useDisclosure();
   const { user, currentOrg, logout, impersonationOriginal, stopImpersonation } = useAuthStore();
+  const { colorScheme, toggleColorScheme } = usePreferencesStore();
   const navigate = useNavigate();
   const location = useLocation();
   const isImpersonating = !!impersonationOriginal;
@@ -108,6 +112,16 @@ export function AppLayout() {
             {currentOrg && (
               <Text size="sm" c="dimmed">{currentOrg.name}</Text>
             )}
+            <Tooltip label={colorScheme === 'dark' ? 'Light mode' : 'Dark mode'}>
+              <ActionIcon
+                variant="default"
+                size="lg"
+                onClick={toggleColorScheme}
+                aria-label="Toggle color scheme"
+              >
+                {colorScheme === 'dark' ? <IconSun size={18} /> : <IconMoon size={18} />}
+              </ActionIcon>
+            </Tooltip>
             <Menu shadow="md" width={220}>
               <Menu.Target>
                 <UnstyledButton>