chore(deps): clear Dependabot alerts — React 19, react-router 8, Vite 7

Bumps both frontend apps (site, admin-ui):
- vite 5.4 -> 7.3.6 (pulls esbuild 0.28, patched dev-server CVEs)
- @vitejs/plugin-react 4 -> 5 (babel 7.29.7)
- postcss 8.4 -> 8.5.25
- react-router-dom 6.27 -> react-router 8.3.0 (package renamed
  upstream in v8; imports swapped in src). Fixes the open-redirect
  advisories with no patched 6.x, plus the RSC CSRF advisory
  (GHSA-qwww-vcr4-c8h2) that only 8.3.0 patches.
- react/react-dom 18.3 -> 19.2 (required by react-router 8)

npm audit: 0 vulnerabilities in both apps. tsc + vite build clean.

DorkNet.Server/admin-ui/src/pages/Dashboard.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useState } from 'react';
-import { Link } from 'react-router-dom';
+import { Link } from 'react-router';
 import { api, get } from '../lib/api';
 import type { Stats } from '../lib/types';
 import { absoluteTime, num, relativeTime } from '../lib/format';

DorkNet.Server/admin-ui/src/pages/Login.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react';
-import { useNavigate, useLocation } from 'react-router-dom';
+import { useNavigate, useLocation } from 'react-router';
 import { api, ApiError } from '../lib/api';
 import { notifyAuthChange, setSession } from '../lib/auth';
 import { BrandMark } from '../components/BrandMark';

DorkNet.Server/admin-ui/src/pages/Players.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useMemo, useState } from 'react';
-import { useSearchParams } from 'react-router-dom';
+import { useSearchParams } from 'react-router';
 import { api, get } from '../lib/api';
 import type { Player, PlayerDetail } from '../lib/types';
 import { absoluteTime, currencyName, num, relativeTime } from '../lib/format';

DorkNet.Server/admin-ui/src/pages/Room.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useMemo, useState } from 'react';
-import { Link, useNavigate, useParams } from 'react-router-dom';
+import { Link, useNavigate, useParams } from 'react-router';
 import { api } from '../lib/api';
 import type { Player, RoomDetail, RoomInstance, RoomRoleGrant } from '../lib/types';
 import { profileImageUrl } from '../lib/types';

DorkNet.Server/admin-ui/src/pages/Rooms.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useState } from 'react';
-import { Link } from 'react-router-dom';
+import { Link } from 'react-router';
 import { api } from '../lib/api';
 import { imageCdnUrl, type Room } from '../lib/types';
 import { useApi } from '../lib/useApi';