style: CSS polish, light mode verification, build fixes

- Add --primary-rgb, --transition-fast, --transition-normal CSS tokens to :root
- Add --primary-rgb override in [data-theme="light"] section
- Fix TS error: remove unused devUser prop from Route elements in App.tsx (API patching via window._devUser makes props redundant)
- Fix TS error: remove unused 'api' import from DevPanel.tsx

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -110,8 +110,8 @@ export default function App() {
 
       <main className={styles.main}>
         <Routes>
-          <Route path="/" element={<DashboardPage key={refreshKey} devUser={devUser} />} />
-          <Route path="/spiele" element={<MatchesPage key={refreshKey} devUser={devUser} />} />
+          <Route path="/" element={<DashboardPage key={refreshKey} />} />
+          <Route path="/spiele" element={<MatchesPage key={refreshKey} />} />
           <Route path="/rangliste" element={<LeaderboardPage key={refreshKey} />} />
           <Route path="/profil" element={<ProfilePage key={refreshKey} />} />
           <Route path="/admin" element={<AdminPage />} />