style: remove premium labels + fix LU badge

- Remove all visible "premium" text labels across the app:
  Navigation sidebar: "Premium Audio" → "Your audio space"
  Header: "Premium Member" → "Membre"
  Login page badge: "Expérience Audio Premium" → "Votre espace audio"
  Dashboard hero badge: "Podcastic Premium" → "Podcastic"
  EpisodeCard fallback author: "Artiste Premium" → empty string

- LU badge: remove Check icon (was rendering as "?" in some envs),
  now shows clean "LU" text only

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

frontend/src/components/Navigation.tsx

@@ -26,7 +26,7 @@ export default function Navigation() {
           </div>
           <div className="flex flex-col">
             <span className="text-xl font-display font-extrabold tracking-tight">PODCASTIC</span>
-            <span className="text-[10px] text-[var(--text-secondary)] font-bold uppercase tracking-[0.2em]">Premium Audio</span>
+            <span className="text-[10px] text-[var(--text-secondary)] font-bold uppercase tracking-[0.2em]">Your audio space</span>
           </div>
         </Link>