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/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',
+    },
+  ),
+);