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/features/ai-chat/services/ai-chat-service.ts

@@ -37,6 +37,17 @@ export async function getAiChatMessages(
   return req.data;
 }
 
+/**
+ * Resolve the chat bound to a document (the current user's most-recent chat
+ * created on that page), or null when there is none. Drives auto-open-on-page.
+ */
+export async function getBoundChat(pageId: string): Promise<string | null> {
+  const req = await api.post<{ chatId: string | null }>("/ai-chat/bound-chat", {
+    pageId,
+  });
+  return req.data.chatId;
+}
+
 /** Rename a chat. */
 export async function renameAiChat(data: {
   chatId: string;