feat(ai): generate page title from content (#199)

Add an AI button in the page byline that generates a note's title from the
live editor content (including unsaved edits) and applies it immediately.

Server: one-shot, non-streaming POST /ai-chat/generate-page-title mirroring
the chat generateTitle path — gated by settings.ai.generative, throttled via
AI_CHAT_THROTTLER, resolves the workspace chat model and returns { title }.
The endpoint never touches the page; the client applies the title through the
existing /pages/update route (which enforces edit permission).

Client: ai-chat-service.generatePageTitle, a useGeneratePageTitle hook that
converts the editor HTML to markdown, calls the endpoint, applies the title
via updateTitle + updatePageData, reflects it in the unfocused title editor,
and broadcasts the UpdateEvent (mirroring TitleEditor.saveTitle). A sparkles
button (GenerateTitleGroup) renders next to dictation, edit-mode + flag gated.

Tests: pure cleanGeneratedTitle helper + controller gate/delegation/error-map.
i18n: en-US + ru-RU strings.

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

apps/client/src/features/ai-chat/services/ai-chat-service.ts

@@ -68,6 +68,19 @@ export async function exportAiChat(
   return req.data.markdown;
 }
 
+/**
+ * Generate a page title from note content (markdown). One-shot, non-streaming
+ * (#199): the server only summarizes the supplied text and returns a suggestion;
+ * it never writes the page. The caller applies the title via /pages/update.
+ */
+export async function generatePageTitle(content: string): Promise<string> {
+  const req = await api.post<{ title: string }>(
+    "/ai-chat/generate-page-title",
+    { content },
+  );
+  return req.data.title;
+}
+
 /**
  * Agent roles API (`/ai-chat/roles`). `list` is available to any workspace
  * member (for the chat-creation picker); create/update/delete are admin-only