feat(ai-chat): auto-open last chat bound to the document (#191)

On opening the floating AI-chat window from the header on a document page,
auto-open the LAST chat bound to that document. Binding reuses the existing
ai_chats.page_id (no migration): the bound chat is the requesting user's
most-recent non-deleted chat created on that page, so a new chat on the page
becomes the bound one for free. Resolution happens only on a genuine
closed -> open transition; the provenance badge deep-link is untouched.

Server: AiChatRepo.findLatestByPage + POST /ai-chat/bound-chat (BoundChatDto),
both read-only and owner/workspace-scoped.
Client: getBoundChat service + useOpenAiChatForCurrentPage hook wired into the
app-header entry point (fail-soft to a fresh chat; draft/role cleared only on a
real switch).
Tests: repo scoping/ordering, controller wiring, and hook behavior.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -10,12 +10,12 @@ 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 { useAtom, useSetAtom } from "jotai";
+import { useAtom } 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 { useOpenAiChatForCurrentPage } from "@/features/ai-chat/hooks/use-open-ai-chat.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";
@@ -38,7 +38,9 @@ export function AppHeader() {
   const toggleDesktop = useToggleSidebar(desktopSidebarAtom);
 
   const [workspace] = useAtom(workspaceAtom);
-  const setAiChatWindowOpen = useSetAtom(aiChatWindowOpenAtom);
+  // Opening from the header auto-opens the document's bound chat (last chat
+  // created on the current page); off a page it keeps the current selection.
+  const openAiChat = useOpenAiChatForCurrentPage();
   // AI chat entry point: only shown when the workspace enables it (A7 gate).
   const aiChatEnabled = workspace?.settings?.ai?.chat === true;
 
@@ -105,7 +107,7 @@ export function AppHeader() {
                 color="dark"
                 size="sm"
                 aria-label={t("AI chat")}
-                onClick={() => setAiChatWindowOpen((v) => !v)}
+                onClick={openAiChat}
               >
                 <IconMessage size={20} />
               </ActionIcon>