Dark mode toggle in settings

crabfit-frontend/src/stores/index.ts

@@ -5,9 +5,11 @@ export const useSettingsStore = create(persist(
   set => ({
     weekStart: 0,
     timeFormat: '12h',
+    theme: 'System',
 
     setWeekStart: weekStart => set({ weekStart }),
     setTimeFormat: timeFormat => set({ timeFormat }),
+    setTheme: theme => set({ theme }),
   }),
   { name: 'crabfit-settings' },
 ));