fix(mui): stop dark/light oscillation (single data-theme owner) + animated ThemeToggle + white avatars

Removed MuiColorSchemeSync: it called MUI setMode which ALSO wrote data-theme, fighting ThemeContext (the other writer) → theme flip-flop loop. ThemeContext is now the sole owner; MUI's cssVariables CSS (scoped by [data-theme]) applies from the attribute, no JS bridge needed. New animated ThemeToggle kit button (sun/moon crossfade+rotate) replaces the static AppShell toggle. DashAttendanceToday avatars now solid-fill + white initials (matches StatCard/activity badges).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/ui/MuiProvider.tsx

@@ -4,13 +4,11 @@ import { LocalizationProvider } from "@mui/x-date-pickers/LocalizationProvider";
 import { AdapterDateFns } from "@mui/x-date-pickers/AdapterDateFns";
 import { cs } from "date-fns/locale";
 import { theme } from "../theme";
-import MuiColorSchemeSync from "./MuiColorSchemeSync";
 
 export default function MuiProvider({ children }: { children: ReactNode }) {
   return (
     <ThemeProvider theme={theme} defaultMode="dark">
       <LocalizationProvider dateAdapter={AdapterDateFns} adapterLocale={cs}>
-        <MuiColorSchemeSync />
         {children}
       </LocalizationProvider>
     </ThemeProvider>