feat: auth-aware UI and role-based access control (#67)

## Summary

- Add `is_admin` column to users table with Alembic migration and a `require_admin` FastAPI dependency that protects all admin-facing write endpoints (games, pokemon, evolutions, bosses, routes CRUD)
- Expose admin status to frontend via user API and update AuthContext to fetch/store `isAdmin` after login
- Make navigation menu auth-aware (different links for logged-out, logged-in, and admin users) and protect frontend routes with `ProtectedRoute` and `AdminRoute` components, preserving deep-linking through redirects
- Fix test reliability: `drop_all` before `create_all` to clear stale PostgreSQL enums from interrupted test runs
- Fix test auth: add `admin_client` fixture and use valid UUID for mock user so tests pass with new admin-protected endpoints

## Test plan

- [x] All 252 backend tests pass
- [ ] Verify non-admin users cannot access admin write endpoints (games, pokemon, evolutions, bosses CRUD)
- [ ] Verify admin users can access admin endpoints normally
- [ ] Verify navigation shows correct links for logged-out, logged-in, and admin states
- [ ] Verify `/admin/*` routes redirect non-admin users with a toast
- [ ] Verify `/runs/new` and `/genlockes/new` redirect unauthenticated users to login, then back after auth

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #67
Co-authored-by: Julian Tabel <juliantabel.jt@gmail.com>
Co-committed-by: Julian Tabel <juliantabel.jt@gmail.com>

frontend/src/App.tsx

@@ -1,5 +1,5 @@
 import { Routes, Route, Navigate } from 'react-router-dom'
-import { Layout } from './components'
+import { Layout, ProtectedRoute, AdminRoute } from './components'
 import { AdminLayout } from './components/admin'
 import {
   AuthCallback,
@@ -35,18 +35,18 @@ function App() {
         <Route path="signup" element={<Signup />} />
         <Route path="auth/callback" element={<AuthCallback />} />
         <Route path="runs" element={<RunList />} />
-        <Route path="runs/new" element={<NewRun />} />
+        <Route path="runs/new" element={<ProtectedRoute><NewRun /></ProtectedRoute>} />
         <Route path="runs/:runId" element={<RunEncounters />} />
         <Route path="runs/:runId/journal/:entryId" element={<JournalEntryPage />} />
         <Route path="genlockes" element={<GenlockeList />} />
-        <Route path="genlockes/new" element={<NewGenlocke />} />
+        <Route path="genlockes/new" element={<ProtectedRoute><NewGenlocke /></ProtectedRoute>} />
         <Route path="genlockes/:genlockeId" element={<GenlockeDetail />} />
         <Route path="stats" element={<Stats />} />
         <Route
           path="runs/:runId/encounters"
           element={<Navigate to=".." relative="path" replace />}
         />
-        <Route path="admin" element={<AdminLayout />}>
+        <Route path="admin" element={<AdminRoute><AdminLayout /></AdminRoute>}>
           <Route index element={<Navigate to="/admin/games" replace />} />
           <Route path="games" element={<AdminGames />} />
           <Route path="games/:gameId" element={<AdminGameDetail />} />