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/editor/components/fixed-toolbar/groups/generate-title-group.tsx

@@ -0,0 +1,39 @@
+import { FC } from "react";
+import { ActionIcon, Tooltip } from "@mantine/core";
+import { IconSparkles } from "@tabler/icons-react";
+import { useTranslation } from "react-i18next";
+import { useGeneratePageTitle } from "@/features/editor/hooks/use-generate-page-title.ts";
+
+interface Props {
+  pageId: string;
+  color?: string;
+  iconSize?: number;
+}
+
+/**
+ * AI "generate title" button (#199). Reads the live editor content and applies a
+ * model-suggested title immediately. Rendered in the page byline, only in edit
+ * mode and when the workspace's generative AI flag is on.
+ */
+export const GenerateTitleGroup: FC<Props> = ({
+  pageId,
+  color = "gray",
+  iconSize = 20,
+}) => {
+  const { t } = useTranslation();
+  const gen = useGeneratePageTitle(pageId);
+
+  return (
+    <Tooltip label={t("Generate title with AI")} withArrow openDelay={250}>
+      <ActionIcon
+        variant="subtle"
+        color={color}
+        aria-label={t("Generate title with AI")}
+        loading={gen.isPending}
+        onClick={() => gen.mutate()}
+      >
+        <IconSparkles size={iconSize} stroke={1.5} />
+      </ActionIcon>
+    </Tooltip>
+  );
+};