feat: v4.0.2 — qualité & robustesse (session expiry UX, validation, mobile, tests)

- Session expiry: redirects ajoutent ?expired=1 → bannière "Session expired" sur login
- Page titles: titre vide → 'Untitled' par défaut (au lieu de chaîne vide)
- Error handling: try/catch dans create_page avec message user-friendly (500)
- Mobile responsive: sidebar slide-in, modales centrées, landing page adaptative
- 10 nouveaux tests de non-régression: landing, register, 404, validation, duplicate
- 143 tests passent (133 + 10)

app/templates/base.html

@@ -6,6 +6,89 @@
   <title>FlowDeck — {% block title_prefix %}Home{% endblock %}</title>
   <link rel="icon" type="image/svg+xml" href="/static/favicon.svg">
   <link rel="stylesheet" href="/static/css/app.css?v=3.0.0">
+  <style>
+    /* ── Mobile responsive (v4.0.2) ── */
+    @media (max-width: 768px) {
+      .sidebar {
+        position: fixed;
+        left: 0;
+        top: 0;
+        bottom: 0;
+        z-index: 100;
+        width: 280px;
+        transform: translateX(-100%);
+        transition: transform .2s ease;
+      }
+      .sidebar.mobile-open {
+        transform: translateX(0);
+        box-shadow: 4px 0 24px rgba(0,0,0,.5);
+      }
+      .sidebar-overlay.visible {
+        position: fixed;
+        inset: 0;
+        background: rgba(0,0,0,.5);
+        z-index: 99;
+      }
+      .main-wrapper {
+        margin-left: 0 !important;
+        width: 100%;
+      }
+      .unified-header {
+        padding: 8px 12px;
+      }
+      .page-editor-wrapper {
+        padding: 16px 8px;
+      }
+      .page-title-block {
+        padding: 16px 8px;
+      }
+      .blocks-container {
+        padding: 0 4px;
+      }
+      .share-dialog, .more-menu, .activity-popover {
+        position: fixed !important;
+        top: 50% !important;
+        left: 50% !important;
+        transform: translate(-50%, -50%) !important;
+        width: 92vw;
+        max-width: 400px;
+        max-height: 80vh;
+        overflow-y: auto;
+      }
+      .landing-nav {
+        padding: 12px 16px;
+      }
+      .hero {
+        padding: 40px 16px 30px;
+      }
+      .hero h1 {
+        font-size: 1.8rem;
+      }
+      .features {
+        grid-template-columns: 1fr;
+        padding: 0 16px 30px;
+      }
+      .library-card-grid {
+        grid-template-columns: 1fr;
+      }
+      .view-tabs {
+        overflow-x: auto;
+        flex-wrap: nowrap;
+      }
+    }
+    @media (max-width: 480px) {
+      .hero h1 {
+        font-size: 1.5rem;
+      }
+      .hero p {
+        font-size: 15px;
+      }
+      .btn {
+        padding: 8px 16px;
+        font-size: 13px;
+      }
+    }
+  </style>
   <script src="/static/js/htmx.min.js"></script>
   <script src="/static/js/alpine.min.js" defer></script>
   <script src="/static/js/sortable.min.js" defer></script>