Add "Mi Token" feature so users can retrieve their Firebase UID

[Copilot]

Users had no way to find their Firebase UID after logging in — loginGoogle() only persisted the display name.

Changes

• loginGoogle() — now also writes r.user.uid to localStorage["uid"] on Google sign-in (both index.html and room.html)
• mostrarToken() — new function that reads the stored UID and copies it to clipboard via the Clipboard API, with an alert() fallback; shows a prompt to log in first if no UID is stored
• "🔑 MI TOKEN" button — added to the header bar in both pages, wired to mostrarToken()

function loginGoogle(){
    const p = new firebase.auth.GoogleAuthProvider();
    firebase.auth().signInWithPopup(p).then(r => {
        localStorage.setItem("username", r.user.displayName);
        localStorage.setItem("uid", r.user.uid);  // ← new
        location.reload();
    });
}

function mostrarToken(){
    const uid = localStorage.getItem("uid");
    if(uid){
        if(navigator.clipboard){
            navigator.clipboard.writeText(uid).then(() => alert("🔑 Token copiado al portapapeles:\n" + uid));
        } else {
            alert("🔑 Tu Token (UID):\n" + uid);
        }
    } else {
        alert("⚠️ Inicia sesión primero con 🔐 LOGIN para obtener tu token.");
    }
}