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/components/dashboard/DashAttendanceToday.tsx

@@ -89,8 +89,8 @@ export default function DashAttendanceToday({
                     height: 36,
                     fontSize: "0.8rem",
                     fontWeight: 700,
-                    bgcolor: isDefault ? "action.hover" : `${color}.light`,
-                    color: isDefault ? "text.secondary" : `${color}.main`,
+                    bgcolor: isDefault ? "grey.600" : `${color}.main`,
+                    color: "common.white",
                   }}
                 >
                   {u.initials || "?"}