feat: add sidebar container styles and collapse animation

frontend/style.css

@@ -373,6 +373,83 @@ body {
   overflow: hidden;
 }
 
+/* ============================================================
+   Session sidebar
+   ============================================================ */
+
+.session-sidebar {
+  width: 200px;
+  min-width: 200px;
+  background: var(--bg-secondary);
+  border-right: 1px solid var(--border-subtle);
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  flex-shrink: 0;
+  transition: width 0.25s ease, min-width 0.25s ease;
+}
+
+.session-sidebar.sidebar--collapsed {
+  width: 0;
+  min-width: 0;
+}
+
+.sidebar-header {
+  display: flex;
+  flex-direction: row;
+  justify-content: space-between;
+  align-items: center;
+  padding: 8px 12px;
+  border-bottom: 1px solid var(--border-subtle);
+  flex-shrink: 0;
+}
+
+.sidebar-title {
+  font-size: 11px;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  color: var(--text-muted);
+}
+
+.sidebar-list {
+  flex: 1;
+  overflow-y: auto;
+  overflow-x: hidden;
+}
+
+.sidebar-collapse-btn {
+  background: none;
+  border: none;
+  color: var(--text-muted);
+  cursor: pointer;
+  font-size: 18px;
+  padding: 2px 6px;
+  border-radius: 4px;
+}
+
+.sidebar-collapse-btn:hover {
+  color: var(--text);
+}
+
+.sidebar-toggle-btn {
+  background: none;
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  width: 36px;
+  height: 36px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  margin-right: 8px;
+  cursor: pointer;
+  color: var(--text-muted);
+}
+
+.sidebar-toggle-btn:hover {
+  border-color: var(--accent);
+}
+
 .terminal-container {
   flex: 1;
   min-width: 0;