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/Select.tsx

@@ -3,7 +3,8 @@
  * @description Generic styled dropdown that replaces native `<select>` elements
  * for consistent cross-platform rendering. Native macOS/Chromium selects reserve
  * checkmark space inconsistently; this control aligns labels, flips above the
- * trigger when viewport space is tight, and marks the chosen option with accent
+ * trigger when space is tight — in its split pane (`data-menu-bounds`, see
+ * `menuRoom`) or else the viewport — and marks the chosen option with accent
  * color plus a Lucide check.
  *
  * ## Accessibility — WAI-ARIA "listbox button"
@@ -74,6 +75,14 @@
 import { useEffect, useId, useRef, useState, type ReactNode } from "react";
 import { ChevronDown, Check } from "lucide-react";
 
+/** Room above and below `r` (the trigger's rect) for a menu it opens: inside
+ *  the nearest `data-menu-bounds` box — a split pane, whose neighbour paints
+ *  over anything crossing its edge — else the viewport. */
+export function menuRoom(el: Element, r: DOMRect): { above: number; below: number } {
+  const box = el.closest("[data-menu-bounds]")?.getBoundingClientRect();
+  return { above: r.top - (box?.top ?? 0), below: (box?.bottom ?? window.innerHeight) - r.bottom };
+}
+
 /** Single option in a {@link Select} list. */
 export interface SelectOption<T extends string> {
   /** Stored value emitted via `onChange`. */
@@ -144,9 +153,11 @@ export function Select<T extends string>({
     const idx = options.findIndex((o) => o.value === value);
     setActive(idx >= 0 ? idx : 0);
     if (buttonRef.current) {
-      const rect = buttonRef.current.getBoundingClientRect();
-      const below = window.innerHeight - rect.bottom;
-      setOpenUp(below < 288 && rect.top > below);
+      const { above, below } = menuRoom(
+        buttonRef.current,
+        buttonRef.current.getBoundingClientRect()
+      );
+      setOpenUp(below < 288 && above > below);
     }
     listRef.current?.focus({ preventScroll: true });
     // eslint-disable-next-line react-hooks/exhaustive-deps -- only on open