feat(phase4): permanent cache-control & asset versioning, canonical nabil stored_session_id & resume, dsh session persistence fix, dsh dialogue/files toggle, and collapsible conv sidebar

static/css/style.css

@@ -420,11 +420,56 @@ body, html {
   flex-direction: column;
   flex-shrink: 0;
   overflow: hidden;
+  transition: width 0.2s cubic-bezier(0.4, 0, 0.2, 1), opacity 0.2s ease, border-color 0.2s ease;
+}
+
+.hub-conv-sidebar.collapsed {
+  width: 0 !important;
+  min-width: 0 !important;
+  border-right: none;
+  opacity: 0;
+  pointer-events: none;
 }
 
 .hub-conv-header {
   padding: 0.75rem;
   border-bottom: 1px solid var(--hub-border-subtle);
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+}
+
+.hub-conv-header .hub-btn-new-chat {
+  flex: 1;
+}
+
+.hub-collapse-conv-btn,
+.hub-expand-conv-btn {
+  background: transparent;
+  border: 1px solid var(--hub-border-subtle);
+  border-radius: var(--hub-radius-sm);
+  color: var(--hub-text-muted);
+  width: 32px;
+  height: 32px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  cursor: pointer;
+  flex-shrink: 0;
+  transition: all 0.15s ease;
+}
+
+.hub-collapse-conv-btn:hover,
+.hub-expand-conv-btn:hover {
+  color: var(--hub-text-primary);
+  background: var(--hub-bg-card-hover);
+  border-color: var(--hub-border-medium);
+}
+
+.hub-chat-header-info {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
 }
 
 .hub-btn-new-chat {