feat(ai-chat): move AI chat icon into the global top bar

Move the AI-chat toggle icon (IconSparkles) from the page header menu
into the global top bar, placed next to the notifications icon. The
"AI chat enabled" gate (workspace.settings.ai.chat) is preserved, and
the icon style is aligned with the neighbouring notifications icon
(subtle, size sm). As a result the entry point is now available on all
routes instead of only on page routes.

- app-header.tsx: render the gated AI-chat ActionIcon before
  NotificationPopover; wire it to aiChatWindowOpenAtom.
- page-header-menu.tsx: remove the old AI icon block and its now-unused
  imports/locals.

apps/client/src/components/layouts/global/app-header.tsx

@@ -1,19 +1,23 @@
 import {
+  ActionIcon,
   Box,
   Group,
   Text,
   Tooltip,
 } from "@mantine/core";
+import { IconSparkles } from "@tabler/icons-react";
 import classes from "./app-header.module.css";
 import { BrandLogo } from "@/components/ui/brand-logo";
 import TopMenu from "@/components/layouts/global/top-menu.tsx";
 import { Link } from "react-router-dom";
 import APP_ROUTE from "@/lib/app-route.ts";
-import { useAtom } from "jotai";
+import { useAtom, useSetAtom } from "jotai";
 import {
   desktopSidebarAtom,
   mobileSidebarAtom,
 } from "@/components/layouts/global/hooks/atoms/sidebar-atom.ts";
+import { aiChatWindowOpenAtom } from "@/features/ai-chat/atoms/ai-chat-atom.ts";
+import { workspaceAtom } from "@/features/user/atoms/current-user-atom.ts";
 import { useToggleSidebar } from "@/components/layouts/global/hooks/hooks/use-toggle-sidebar.ts";
 import SidebarToggle from "@/components/ui/sidebar-toggle-button.tsx";
 import { useTranslation } from "react-i18next";
@@ -38,6 +42,11 @@ export function AppHeader() {
   const [desktopOpened] = useAtom(desktopSidebarAtom);
   const toggleDesktop = useToggleSidebar(desktopSidebarAtom);
 
+  const [workspace] = useAtom(workspaceAtom);
+  const setAiChatWindowOpen = useSetAtom(aiChatWindowOpenAtom);
+  // AI chat entry point: only shown when the workspace enables it (A7 gate).
+  const aiChatEnabled = workspace?.settings?.ai?.chat === true;
+
   const items = links.map((link) => (
     <Link key={link.label} to={link.link} className={classes.link}>
       {t(link.label)}
@@ -105,6 +114,19 @@ export function AppHeader() {
         </div>
 
         <Group px={"xl"} wrap="nowrap">
+          {aiChatEnabled && (
+            <Tooltip label={t("AI chat")} withArrow>
+              <ActionIcon
+                variant="subtle"
+                color="dark"
+                size="sm"
+                aria-label={t("AI chat")}
+                onClick={() => setAiChatWindowOpen((v) => !v)}
+              >
+                <IconSparkles size={20} />
+              </ActionIcon>
+            </Tooltip>
+          )}
           <NotificationPopover />
           <TopMenu />
         </Group>