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/admin/pages/AttendanceLocation.tsx

@@ -6,10 +6,25 @@ import Forbidden from "../components/Forbidden";
 import { useNavigate, useParams, Link as RouterLink } from "react-router-dom";
 import Box from "@mui/material/Box";
 import Typography from "@mui/material/Typography";
+import { styled } from "@mui/material/styles";
 
 import L from "leaflet";
 import "leaflet/dist/leaflet.css";
 
+// Leaflet map container (was the .attendance-location-map rule). zIndex:0
+// creates a stacking context so Leaflet's internal z-indexes stay contained.
+const LocationMap = styled("div")(({ theme }) => ({
+  height: "400px",
+  borderRadius: "8px",
+  marginBottom: "1.5rem",
+  background: theme.vars!.palette.background.paper,
+  position: "relative",
+  zIndex: 0,
+  "@media (max-width:640px)": {
+    height: "clamp(220px, 45vh, 400px)",
+  },
+}));
+
 import { formatDate, formatTime } from "../utils/attendanceHelpers";
 import {
   attendanceLocationOptions,
@@ -209,10 +224,8 @@ export default function AttendanceLocation() {
           {record.user_name} — {formatDate(record.shift_date)}
         </Typography>
 
-        {/* Leaflet map — kept entirely as-is */}
-        {hasAnyLocation && (
-          <div ref={mapRef} className="attendance-location-map" />
-        )}
+        {/* Leaflet map */}
+        {hasAnyLocation && <LocationMap ref={mapRef} />}
 
         {/* Location detail grid */}
         <Box