fix: add close buttons to mobile sidebars

- RightSidebar: added onCollapse prop and close button in header

- LeftSidebar: wired existing onCollapse prop in App.tsx

- CSS: show .leftbar-toggle and .rightbar-close on mobile

- Both buttons call setMobile{Left,Right}Open(false) to close overlays

frontend/src/styles.css

@@ -1273,6 +1273,23 @@ a:hover { text-decoration: underline; }
   body.drawer-open { overflow: auto !important; }
   /* Mobile sidebars: scrollable content, visible labels */
   .leftbar { overflow-y: auto; -webkit-overflow-scrolling: touch; }
+  .leftbar-toggle { display: grid !important; }
+  .rightbar-close { display: grid !important; }
+  .rightbar-close {
+    position: absolute;
+    top: 6px;
+    left: 6px;
+    width: 32px;
+    height: 32px;
+    border: none;
+    border-radius: 6px;
+    background: transparent;
+    color: var(--color-text);
+    cursor: pointer;
+    z-index: 10;
+  }
+  .rightbar-close svg { width: 18px; height: 18px; }
+  .rightbar-close:hover { background: var(--color-surface-3); }
   .leftbar-title { display: block; }
   .tool-section-label { display: block; padding: 0 6px 6px; }
   .tool-btn-label { display: block; }