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>

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>,
 );

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>
 

frontend/src/stores/preferencesStore.ts

@@ -0,0 +1,26 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+type ColorScheme = 'light' | 'dark';
+
+interface PreferencesState {
+  colorScheme: ColorScheme;
+  toggleColorScheme: () => void;
+  setColorScheme: (scheme: ColorScheme) => void;
+}
+
+export const usePreferencesStore = create<PreferencesState>()(
+  persist(
+    (set) => ({
+      colorScheme: 'light',
+      toggleColorScheme: () =>
+        set((state) => ({
+          colorScheme: state.colorScheme === 'light' ? 'dark' : 'light',
+        })),
+      setColorScheme: (scheme) => set({ colorScheme: scheme }),
+    }),
+    {
+      name: 'ledgeriq-preferences',
+    },
+  ),
+);