refactor(css): migrate Login + file icons off legacy vars, delete variables.css

Phase 4 (final) of the "fully MUI" cleanup — no custom stylesheets remain
in src/admin (only the third-party quill.snow.css + leaflet.css imports).

- Login.tsx: every var(--glass-*/bg-*/text-*/accent-*/orb-*/border-*/
  font-*/transition) in its sx/inline styles is now an MUI token
  (bgcolor/color/borderColor: "background.paper" / "text.secondary" /
  "divider" / "primary.main"), a channel-alpha (rgba(var(--mui-palette-
  primary-mainChannel) / a)) for tints, or theme.applyStyles("dark", …)
  for the glass-card shadow. The card sx is now a (theme)=>({}) callback.
- Restores the orb drift animation via the MUI keyframes() helper — the
  @keyframes float had been dropped in Phase 1 (the dead-CSS scan missed
  its use inside an sx animation string); recovered verbatim from git.
- ProjectFileManager: the fallback file-icon color uses
  var(--mui-palette-text-secondary).
- variables.css deleted; its import removed from AdminApp (last one).

tsc -b --noEmit + npm run build clean. applyStyles dark-scheme output and
the channel-alpha pattern were both verified live in Chrome via the Quill
editor, which uses the identical constructs.

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

src/admin/AdminApp.tsx

@@ -11,7 +11,6 @@ import MuiProvider from "./ui/MuiProvider";
 import { LoadingState } from "./ui";
 import Login from "./pages/Login";
 import Dashboard from "./pages/Dashboard";
-import "./variables.css";
 
 const Users = lazy(() => import("./pages/Users"));
 const Attendance = lazy(() => import("./pages/Attendance"));