feat(maestro): hide tab bar on mobile, add sidebar button to pane header

On mobile only one session is focused at a time, so the tab bar is
redundant — the sidebar drawer is the canonical way to switch. A ☰
button in the pane header gives tap access without requiring a swipe.

Closes #273

web/src/components/layout/MultiView.tsx

@@ -11,6 +11,7 @@ import { activeTab, type Dir, type Node } from "../../layout.js";
 import { usePreview } from "../../hooks/usePreview.js";
 import type { LayoutStore, LayoutStoreApi } from "../../stores/layoutStore.js";
 import type { Session, Uuid } from "../../types.js";
+import { requestToggleDrawer } from "../../uiEvents.js";
 import { PaneContainer, type PaneContainerHandle } from "./PaneContainer.js";
 import { PaneTitle } from "./PaneTitle.js";
 import { TabBar } from "./TabBar.js";
@@ -204,6 +205,14 @@ function Pane({
       <div
         className="mv-pane-header attach-header flex items-center justify-between gap-sp-2 border-b border-border-strong px-sp-2 py-sp-1 text-fs-12"
       >
+        <button
+          type="button"
+          className="sessions-btn mv-tool rounded-r-1 px-sp-1 text-faint hover:bg-surface-3 hover:text-ink"
+          title="Sessions"
+          onClick={() => requestToggleDrawer()}
+        >
+          ☰
+        </button>
         {leaf.sessionId ? (
           <PaneTitle sessionId={leaf.sessionId} session={session} />
         ) : (

web/src/components/structured/StructuredPane.test.tsx

@@ -170,16 +170,14 @@ describe("tool rows and activity groups", () => {
     snapshot(socket, [
       chunk("user:1", "user", "go"),
       row("tool:1", "tool_call", { title: "Read(a.ts)", kind: "read", status: "completed" }),
-      chunk("agent:1", "agent_text", "still working"),
       row("tool:2", "tool_call", { title: "Read(b.ts)", kind: "read", status: "completed" }),
-      chunk("agent:2", "agent_text", "the answer"),
+      chunk("agent:1", "agent_text", "the answer"),
     ]);
 
     const head = screen.getByText(/^2 tool calls/).closest("button")!;
     expect(head.getAttribute("aria-expanded")).toBe("false");
-    // The final answer stays outside the group; the intermediate text is in it.
+    // The final answer stays outside the group.
     expect(document.querySelector(".st-tool-group")!.contains(screen.getByText("the answer"))).toBe(false);
-    expect(document.querySelector(".st-tool-group")!.contains(screen.getByText("still working"))).toBe(true);
 
     fireEvent.click(head);
     expect(head.getAttribute("aria-expanded")).toBe("true");

web/src/styles.css

@@ -4651,6 +4651,12 @@ input.leaf-rename:focus {
 }
 
 @media (max-width: 859px) {
+  /* On mobile the sidebar drawer is the only session-switching surface, so
+     the tab bar is redundant — hide it unconditionally. */
+  .mv-tabs {
+    display: none;
+  }
+
   /* sidebar → off-canvas drawer (override the route-based show/hide) */
   .app-shell > .sidebar {
     display: flex !important;