refactor(css): migrate base.css to MUI GlobalStyles and Leaflet to styled()

Phase 2 of the "fully MUI" cleanup — eliminates two more stylesheets.

- base.css -> src/admin/GlobalStyles.tsx: the reset, typography,
  scrollbar/::selection, theme cross-fade timing and all utility
  classes now live in a single theme-aware MUI <GlobalStyles>, rendered
  inside MuiProvider so every rule resolves against theme.vars (verified
  reactive in both schemes). base.css deleted.
- attendance.css -> a styled("div") LocationMap in AttendanceLocation;
  attendance.css deleted.
- App.tsx bootstrap loader is now fully self-contained (inline spinner
  + keyframes, theme-aware background read from the data-theme attr),
  since it renders before MuiProvider/GlobalStyles mount — it no longer
  depends on the removed .admin-spinner / var(--bg-primary).

tsc -b --noEmit and npm run build clean; verified in Chrome (light +
dark): body bg, text, fonts, .text-warning/.link-accent all correct.

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

src/App.tsx

@@ -2,7 +2,13 @@ import { Suspense } from "react";
 import { Routes, Route } from "react-router-dom";
 import AdminApp from "./admin/AdminApp";
 
+// Bootstrap loader: renders before MuiProvider (and its global styles) mount,
+// so it must be fully self-contained — no theme vars, no global CSS classes.
+// Background follows the persisted theme attribute set by ThemeContext.
 function AdminLoader() {
+  const light =
+    typeof document !== "undefined" &&
+    document.documentElement.getAttribute("data-theme") === "light";
   return (
     <div
       style={{
@@ -10,10 +16,20 @@ function AdminLoader() {
         display: "flex",
         alignItems: "center",
         justifyContent: "center",
-        background: "var(--bg-primary)",
+        background: light ? "#f4f3f1" : "#0f0f0f",
       }}
     >
-      <div className="admin-spinner" />
+      <style>{"@keyframes boha-boot-spin{to{transform:rotate(360deg)}}"}</style>
+      <div
+        style={{
+          width: 32,
+          height: 32,
+          border: "2px solid #d63031",
+          borderTopColor: "transparent",
+          borderRadius: "50%",
+          animation: "boha-boot-spin 0.8s linear infinite",
+        }}
+      />
     </div>
   );
 }