feat: replace trophy emoji with official FIFA WM 2026 logo

White SVG logo for dark mode, colored SVG for light mode.
Logo switches automatically based on theme.
Sourced from football-logos.cc (SVG, ~4.5KB each).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/App.tsx

@@ -77,8 +77,12 @@ export default function App() {
       <header className={styles.header}>
         <div className={styles.headerInner}>
           <div className={styles.logo}>
-            <span className={styles.logoFlag}>🏆</span>
-            <span className={styles.logoText}>WM 2026 Tippspiel</span>
+            <img
+              src={theme === 'dark' ? '/assets/wm2026-logo-white.svg' : '/assets/wm2026-logo.svg'}
+              alt="FIFA WM 2026"
+              className={styles.logoImg}
+            />
+            <span className={styles.logoText}>Tippspiel</span>
             {IS_DEV && (
               <span className={styles.devBadge}>DEV · User {devUser}</span>
             )}