feat: frontend modernization with HTMX, Alpine.js and Plyr (Phase 3)

- Integrated HTMX for server-driven UI updates and fragments
- Adopted Alpine.js for global reactive state and tab management
- Replaced legacy player with Plyr.io for premium streaming experience
- Implemented real-time download polling via HTMX
- Added server-sent Toast notification system
- Fixed navigation and authentication scoping issues

static/js/auth.js

@@ -91,8 +91,12 @@ async function checkAuth() {
 
         if (response.ok) {
             const data = await response.json();
-            showUserInfo(data.user);
-            showMainContent();
+            
+            // Dispatch event for Alpine.js global state
+            window.dispatchEvent(new CustomEvent('auth-success', { 
+                detail: { username: data.user.full_name || data.user.username } 
+            }));
+            
             return true;
         } else {
             // Token invalid, remove it and redirect
@@ -116,45 +120,6 @@ function redirectToLogin() {
     }
 }
 
-// Show user info when authenticated
-function showUserInfo(user) {
-    const userInfo = document.getElementById('userInfo');
-    const loginPrompt = document.getElementById('loginPrompt');
-    const mainTabs = document.getElementById('mainTabs');
-    const currentUser = document.getElementById('currentUser');
-
-    if (userInfo) userInfo.style.display = 'flex';
-    if (loginPrompt) loginPrompt.style.display = 'none';
-    if (mainTabs) mainTabs.style.visibility = 'visible';
-    if (currentUser) currentUser.textContent = user.full_name || user.username;
-}
-
-// Show main content (only when authenticated)
-function showMainContent() {
-    const mainContent = document.getElementById('main-content');
-    if (mainContent) mainContent.style.display = 'block';
-}
-
-// Hide main content (when not authenticated)
-function hideMainContent() {
-    const mainContent = document.getElementById('main-content');
-    if (mainContent) mainContent.style.display = 'none';
-}
-
-// Show login prompt when not authenticated (not used anymore - we redirect instead)
-function showLoginPrompt() {
-    const userInfo = document.getElementById('userInfo');
-    const loginPrompt = document.getElementById('loginPrompt');
-    const mainTabs = document.getElementById('mainTabs');
-
-    if (userInfo) userInfo.style.display = 'none';
-    if (loginPrompt) loginPrompt.style.display = 'block';
-    if (mainTabs) mainTabs.style.visibility = 'hidden';
-
-    // Hide main content
-    hideMainContent();
-}
-
 // Handle logout
 async function handleLogout() {
     if (!confirm('Êtes-vous sûr de vouloir vous déconnecter?')) {