feat: v2.6.0 — Chat UI composer (+ menu, project files, activity panel, permission mode, Stop), git on a workspace's extra folders (#70), long paste as a file (#68), S3 server mode (experimental)

Chat UI:
- Each Claude/Codex session keeps its own chat/terminal view; Settings only
  sets how new sessions open.
- Composer + menu: upload, project files and folders (#54, also dragged from
  the Files/Changes tree), context files, recent uploads, skills, plugins.
- Activity side panel (progress, outputs, files read, uploads, memory,
  skills, connectors) with a file preview; edited-file counts per turn.
- Permission-mode chip (Shift+Tab steps, read back from the pane:
  server/lib/pane-mode.js, GET|POST /api/run/:id/permission-mode).
- Stop (Esc) while the main agent works; the status is checked per agent
  (with session_id /api/agents ignores its status filter) and re-read every
  5 s while Stop shows.
- Dropped images shown in the conversation (GET /api/run/drop/:name, the
  requester's own drop dir), full-size viewer.
- A plain-text paste over 5,000 characters is attached as pasted-text.txt,
  so no prompt reaches tmux/argv limits (#68).

Changes panel: every /api/lanes/:id/git* route takes ?source=<lane_sources
id> (sourceFolder; ENOSOURCE otherwise) and a folder picker switches the
whole panel to a workspace's extra folder (#70).

Server mode S3 (one Linux user per account) — experimental, verified with
fake sudo plus a P6 managed-settings spike on a test account:
- member tmux listing runs in the background (cached, 15 s => unknown);
- transcript snapshot copy is async, chunked, temp file + rename;
- Express Layer patch only on Express 4;
- tmux capturePane/sendKey go to the member's own server.

Docs: the multi-user design moved in as docs/plans/server-mode/README.md
(the AgentHub-Server repo is gone); S3-PLAN P6 result; LANES/API/openapi.

client/src/components/settings/__tests__/AppearanceSettings.test.tsx

@@ -88,16 +88,17 @@ describe("AppearanceSettings", () => {
   it("picks the terminal font, JetBrains Mono by default, and warns on System", async () => {
     const user = userEvent.setup();
     render(<AppearanceSettings />);
-    const group = screen.getByRole("radiogroup", { name: "Terminal font" });
-    expect(within(group).getByRole("radio", { name: "JetBrains Mono" })).toHaveAttribute(
-      "aria-checked",
-      "true"
-    );
-    await user.click(within(group).getByRole("radio", { name: "Maple Mono (rounded)" }));
+    const trigger = screen.getByRole("button", { name: /Terminal font/ });
+    expect(trigger).toHaveTextContent("JetBrains Mono");
+    const pick = async (name: string) => {
+      await user.click(screen.getByRole("button", { name: /Terminal font/ }));
+      await user.click(screen.getByRole("option", { name }));
+    };
+    await pick("Maple Mono (rounded)");
     expect(localStorage.getItem("agenthub.terminal.font")).toBe("maple");
     expect(screen.getByTestId("terminal-font-preview").style.fontFamily).toContain("Maple Mono");
     expect(screen.queryByText(/may lack some Vietnamese letters/)).toBeNull();
-    await user.click(within(group).getByRole("radio", { name: "System font" }));
+    await pick("System font");
     expect(localStorage.getItem("agenthub.terminal.font")).toBe("system");
     expect(screen.getByText(/may lack some Vietnamese letters/)).toBeInTheDocument();
   });