feat: Improve chat page filters layout

- Works filter section: Increase max-height from 250px to 70vh (full screen)
- Context RAG section: Closed by default (display: none)
- Mobile responsive: Adjust works filter to 50vh on mobile
- Enhances visibility of available works at page load

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

generations/library_rag/templates/chat.html

@@ -665,7 +665,7 @@
 
     .works-filter-content {
         padding: 0.75rem 1rem 1rem 1rem;
-        max-height: 250px;
+        max-height: 70vh;
         overflow-y: auto;
     }
 
@@ -818,7 +818,7 @@
         }
 
         .works-filter-content {
-            max-height: 150px;
+            max-height: 50vh;
         }
 
         .sidebar-panel .context-sidebar {
@@ -918,9 +918,9 @@
                 <span>📚</span>
                 <span>Contexte RAG</span>
             </div>
-            <button class="collapse-btn" id="collapse-btn" title="Réduire">▼</button>
+            <button class="collapse-btn" id="collapse-btn" title="Développer">▲</button>
         </div>
-        <div class="sidebar-content" id="sidebar-content">
+        <div class="sidebar-content" id="sidebar-content" style="display: none;">
             <div class="sidebar-empty">
                 Posez une question pour voir le contexte utilisé
             </div>