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/PathPickButton.tsx

@@ -12,7 +12,7 @@
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
-import { useState } from "react";
+import { useState, type Ref } from "react";
 import { useTranslation } from "react-i18next";
 import { FolderOpen } from "lucide-react";
 import { api } from "../lib/api";
@@ -27,12 +27,15 @@ export function PathPickButton({
   start,
   onPick,
   showLabel = false,
+  buttonRef,
 }: {
   kind?: "dir" | "file";
   /** The field's current value — the picker opens there. */
   start?: string;
   onPick: (path: string) => void;
   showLabel?: boolean;
+  /** Lets a field beside it open the same picker (a click on an empty path). */
+  buttonRef?: Ref<HTMLButtonElement>;
 }) {
   const { t } = useTranslation(["lanes"]);
   const [busy, setBusy] = useState(false);
@@ -61,6 +64,7 @@ export function PathPickButton({
   return (
     <>
       <button
+        ref={buttonRef}
         type="button"
         onClick={pick}
         disabled={busy}