feat: redesign complet de l'application basé sur les maquettes Stitch

Refonte du design system (CSS variables, composants glass/card/button/badge),
nouvelle page PodcastDetails avec route /podcast/:id,
lecteur audio immersif avec visualiseur audio,
animations, thème dark/light, responsive mobile/desktop.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/index.html

@@ -1,17 +1,17 @@
 <!doctype html>
-<html lang="en">
+<html lang="fr">
   <head>
     <meta charset="UTF-8" />
     <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
-    <meta name="description" content="Podcastic - Stream your favorite podcasts" />
+    <meta name="description" content="Podcastic - Votre univers audio premium" />
     <meta name="theme-color" content="#020617" />
     <link rel="preconnect" href="https://fonts.googleapis.com">
     <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
-    <link href="https://fonts.googleapis.com/css2?family=Montserrat:wght@700;800&family=Outfit:wght@300;400;500;600;700&display=swap" rel="stylesheet">
-    <title>Podcastic - Premium Podcast Streaming</title>
+    <link href="https://fonts.googleapis.com/css2?family=Montserrat:wght@600;700;800&family=Outfit:wght@300;400;500;600;700&display=swap" rel="stylesheet">
+    <title>Podcastic - Streaming Audio Premium</title>
   </head>
-  <body>
+  <body class="dark-theme">
     <div id="root"></div>
     <script type="module" src="/src/main.tsx"></script>
   </body>