refactor(ai): drop Generative AI flag, gate title generation on AI chat

Remove the separate, un-toggleable `settings.ai.generative` workspace flag
(and its write-side alias `generativeAi`) along with the dead "Ask AI"
generative editor menu, and re-gate the AI page-title generation on the
general AI chat flag (`settings.ai.chat`) — the same toggle that enables
the chat agent and the chat stream endpoint.

Why: the `generative` flag had no UI toggle (its switch was already removed,
leaving orphaned i18n strings), so the title-generation button was
unreachable on self-hosted. The "Ask AI" menu was dead — its atom was never
rendered. Consolidating onto the AI chat flag makes the title button follow
the one AI switch users actually have.

Changes:
- server: title-gen endpoint gate generative -> chat (ai-chat.controller.ts);
  remove generativeAi from update DTO and workspace service (update block,
  delete line, cloud default now { ai: { chat: true } }); fix repo comment;
  migrate generate-page-title spec assertions generative -> chat.
- client: title-gen gate -> settings.ai.chat (full-editor.tsx); remove the
  dead Ask AI button + showAiMenu wiring from bubble-menu; remove AskAiGroup
  usage/import and commented block from fixed-toolbar; delete ask-ai-group.tsx;
  remove showAiMenuAtom; drop generative/generativeAi from workspace types.
- i18n: remove 3 orphaned generative-AI keys from all 12 locales.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/editor/full-editor.tsx

@@ -77,9 +77,9 @@ export function FullEditor({
   const [user] = useAtom(userAtom);
   const workspace = useAtomValue(workspaceAtom);
   const isDictationEnabled = workspace?.settings?.ai?.dictation === true;
-  // AI title generation reuses the generative AI flag (same gate as the on-page
-  // generative menu); the server enforces it too (#199).
-  const isTitleGenEnabled = workspace?.settings?.ai?.generative === true;
+  // AI title generation is gated by the general AI chat flag (the same toggle
+  // that enables the chat agent); the server enforces it too (#199).
+  const isTitleGenEnabled = workspace?.settings?.ai?.chat === true;
   const fullPageWidth = user.settings?.preferences?.fullPageWidth;
   const editorToolbarEnabled =
     user.settings?.preferences?.editorToolbar ?? false;
@@ -254,7 +254,7 @@ function PageByline({
         {showDictation && editor && (
           <DictationGroup editor={editor} color="gray" iconSize={20} />
         )}
-        {/* Shown only in edit mode when the workspace's generative AI flag is on,
+        {/* Shown only in edit mode when the workspace's AI chat flag is on,
             so AI title generation stays reachable from the byline (#199). */}
         {showTitleGen && (
           <GenerateTitleGroup pageId={pageId} color="gray" iconSize={20} />