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/hooks/useTheme.ts

@@ -7,11 +7,15 @@
  * themselves are CSS-variable blocks in `index.css`. State is never cached in
  * the module — every instance reads storage + `matchMedia` through
  * `useSyncExternalStore`, so the Sidebar and Settings can never disagree, and
- * another tab's change arrives through the `storage` event.
+ * another tab's change arrives through the `storage` event. Every change is also
+ * saved to the account (`/api/me/prefs`, same keys); a sign-in loads it back
+ * through `lib/userPrefs.ts` (`hydrate`), while the login page wears the OS's
+ * light/dark (`applySystemTheme`) because nobody has chosen yet.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
 
 import { useCallback, useLayoutEffect, useSyncExternalStore } from "react";
+import { api } from "../lib/api";
 
 export type Theme = "dark" | "light";
 export type ThemeMode = Theme | "system";
@@ -19,7 +23,7 @@ export type ThemeMode = Theme | "system";
 /** Named themes per mode, in picker order. `default` is the stock palette. */
 export const THEMES = {
   dark: ["default", "darcula", "apple-dark", "orca-dark"],
-  light: ["default", "apple-light", "canvas", "orca-light"],
+  light: ["default", "canvas", "apple-light", "orca-light"],
 } as const satisfies Record<Theme, readonly string[]>;
 
 const MODE_KEY = "theme";
@@ -35,13 +39,20 @@ function read(key: string): string | null {
   }
 }
 
-function write(key: string, value: string): void {
+function store(key: string, value: string): void {
   try {
     localStorage.setItem(key, value);
   } catch {
     /* ignore quota / disabled storage */
   }
+}
+
+/** A user's own choice: this browser, every reader, and the account. */
+function write(key: string, value: string): void {
+  store(key, value);
   window.dispatchEvent(new Event(CHANGE_EVENT));
+  // This browser keeps it even when the save fails (offline, signed out).
+  api.me.savePrefs({ [key]: value }).catch(() => {});
 }
 
 function systemQuery(): MediaQueryList | undefined {
@@ -86,6 +97,17 @@ function apply(resolved: Theme, name: string): void {
   else root.dataset.theme = name;
 }
 
+/** Paint what this browser stores (index.html's job at page load). */
+export function applyStoredTheme(): void {
+  const [, resolved, dark, light] = getSnapshot().split("|") as [ThemeMode, Theme, string, string];
+  apply(resolved, resolved === "dark" ? dark : light);
+}
+
+/** The login page: the OS's light/dark in the stock palette. */
+export function applySystemTheme(): void {
+  apply(systemQuery()?.matches === false ? "light" : "dark", "default");
+}
+
 /**
  * Read/write the dashboard's appearance.
  * @returns `theme` — the RESOLVED mode (`system` already answered); `mode` — what