feat(web): add collapse toggle for the chat side panel

The right-hand chat panel (model picker + session list) is a fixed 240px
column on desktop with no way to hide it. Add a collapse button (X) in the
panel header and a floating 'panel' button over the terminal to reopen it,
mirroring the collapsible app sidebar. The choice is persisted in
localStorage (hermes-chat-panel-collapsed) so it survives reloads.

(cherry picked from commit 854f1325a5490bcc968723a82746c939118f6b67)

web/src/pages/ChatPage.test.tsx

@@ -158,6 +158,25 @@ type CloseEventLike = {
 let container: HTMLDivElement;
 let root: Root;
 
+// jsdom runs without an origin here (per-file @vitest-environment jsdom on a
+// node-default config), so localStorage is undefined. Stub it so components
+// that persist UI state (side panel collapse) can be exercised.
+const localStorageMock = (() => {
+  let store: Record<string, string> = {};
+  return {
+    getItem: (key: string) => store[key] ?? null,
+    setItem: (key: string, value: string) => {
+      store[key] = String(value);
+    },
+    removeItem: (key: string) => {
+      delete store[key];
+    },
+    clear: () => {
+      store = {};
+    },
+  };
+})();
+
 async function render(ui: ReactNode) {
   container = document.createElement("div");
   document.body.append(container);
@@ -220,6 +239,8 @@ beforeEach(() => {
     },
   });
   sessionStorage.clear();
+  vi.stubGlobal("localStorage", localStorageMock);
+  localStorageMock.clear();
 });
 
 afterEach(async () => {
@@ -250,6 +271,54 @@ describe("ChatPage", () => {
   });
 });
 
+describe("ChatPage side panel collapse", () => {
+  async function renderChat() {
+    const { default: ChatPage } = await import("./ChatPage");
+    await render(
+      <MemoryRouter initialEntries={["/chat"]}>
+        <ChatPage isActive />
+      </MemoryRouter>,
+    );
+  }
+
+  it("collapses the desktop side panel and persists the choice", async () => {
+    localStorage.clear();
+    await renderChat();
+    await vi.waitFor(() => expect(FakeWebSocket.instances).toHaveLength(1));
+
+    const collapseButton = container.querySelector(
+      '[aria-label="Collapse chat side panel"]',
+    );
+    expect(collapseButton).not.toBeNull();
+
+    await act(async () => {
+      collapseButton!.dispatchEvent(
+        new MouseEvent("click", { bubbles: true }),
+      );
+    });
+
+    expect(localStorage.getItem("hermes-chat-panel-collapsed")).toBe("1");
+    expect(
+      container.querySelector('[aria-label="Collapse chat side panel"]'),
+    ).toBeNull();
+    expect(
+      container.querySelector('[aria-label="Show chat side panel"]'),
+    ).not.toBeNull();
+
+    // Reopening restores the panel and clears the persisted flag.
+    await act(async () => {
+      container
+        .querySelector('[aria-label="Show chat side panel"]')!
+        .dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+
+    expect(localStorage.getItem("hermes-chat-panel-collapsed")).toBe("0");
+    expect(
+      container.querySelector('[aria-label="Collapse chat side panel"]'),
+    ).not.toBeNull();
+  });
+});
+
 // The gated-mode ticket request runs before any socket exists, so a rejection
 // or a hang emits no `close` event and never arms PTY_CONNECTING_TIMEOUT_MS
 // (that timer is set after `new WebSocket`). Without its own deadline the tab

web/src/pages/ChatPage.tsx

@@ -290,6 +290,19 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
   // tabs because the dep wouldn't change on tab switch.
   const [mobilePanelOpenRaw, setMobilePanelOpenRaw] = useState(false);
   const mobilePanelOpen = isActive && mobilePanelOpenRaw;
+
+  // Collapse toggle for the desktop chat side panel (model + sessions),
+  // persisted in localStorage so the choice survives reloads.
+  const [chatPanelCollapsed, setChatPanelCollapsed] = useState(
+    () => localStorage.getItem("hermes-chat-panel-collapsed") === "1",
+  );
+  const toggleChatPanel = useCallback(() => {
+    setChatPanelCollapsed((prev) => {
+      const next = !prev;
+      localStorage.setItem("hermes-chat-panel-collapsed", next ? "1" : "0");
+      return next;
+    });
+  }, []);
   const { setEnd, setTitle } = usePageHeader();
   const [sessionTitleState, setSessionTitleState] = useState<{
     scope: string;
@@ -1717,15 +1730,53 @@ export default function ChatPage({ isActive = true }: { isActive?: boolean }) {
               </span>
             </span>
           </Button>
+
+          {chatPanelCollapsed && (
+            <Button
+              ghost
+              onClick={toggleChatPanel}
+              title="Show side panel (model + sessions)"
+              aria-label="Show chat side panel"
+              className={cn(
+                "absolute z-10",
+                "normal-case tracking-normal font-normal",
+                "rounded border border-current/30",
+                "bg-black/20",
+                "opacity-70 hover:opacity-100 hover:border-current/60",
+                "transition-opacity duration-150",
+                "top-2 right-2 px-2 py-1 text-xs sm:top-3 sm:right-3",
+              )}
+              style={{ color: terminalFg }}
+            >
+              <span className="inline-flex items-center gap-1">
+                <PanelRight className="h-3 w-3 shrink-0" />
+                <span className="hidden min-[400px]:inline tracking-wide">
+                  panel
+                </span>
+              </span>
+            </Button>
+          )}
         </div>
 
-        {!narrow && (
+        {!narrow && !chatPanelCollapsed && (
           <div
             id="chat-side-panel"
             role="complementary"
             aria-label={modelToolsLabel}
             className="flex min-h-0 shrink-0 flex-col gap-3 overflow-hidden lg:h-full lg:w-60"
           >
+            <div className="flex h-8 shrink-0 items-center justify-end pr-1">
+              <Button
+                ghost
+                size="icon"
+                onClick={toggleChatPanel}
+                aria-label="Collapse chat side panel"
+                title="Collapse side panel"
+                className="text-text-secondary hover:text-midground"
+              >
+                <X />
+              </Button>
+            </div>
             {/* Model picker — keeps the rail thin. */}
             <div className="shrink-0">
               <ChatSidebar