feat: add bottom navigation bar (mobile-first)

Fixed bottom nav with Home/Spiele/Rangliste/Profil tabs.
Desktop keeps header nav. Admin hidden behind gear icon.
Main content padded to avoid overlap with bottom nav.

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

frontend/src/global.d.ts

@@ -4,8 +4,14 @@ interface Window {
 
 interface ImportMetaEnv {
   readonly VITE_TEST_MODE?: string;
+  readonly DEV: boolean;
 }
 
 interface ImportMeta {
   readonly env: ImportMetaEnv;
 }
+
+declare module '*.module.css' {
+  const classes: { readonly [key: string]: string };
+  export default classes;
+}