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/components/fixed-toolbar/fixed-toolbar.tsx

@@ -12,8 +12,6 @@ import { MediaGroup } from "./groups/media-group";
 import { QuickInsertsGroup } from "./groups/quick-inserts-group";
 import { MoreInsertsGroup } from "./groups/more-inserts-group";
 import { HistoryGroup } from "./groups/history-group";
-import { AskAiGroup } from "./groups/ask-ai-group";
-import { workspaceAtom } from "@/features/user/atoms/current-user-atom";
 import classes from "./fixed-toolbar.module.css";
 
 type FixedToolbarProps = {
@@ -28,8 +26,6 @@ export const FixedToolbar: FC<FixedToolbarProps> = ({
   const editorFromAtom = useAtomValue(pageEditorAtom);
   const editor = editorProp ?? editorFromAtom;
   const state = useToolbarState(editor);
-  const workspace = useAtomValue(workspaceAtom);
-  const isGenerativeAiEnabled = workspace?.settings?.ai?.generative === true;
 
   if (!editor || !state) return null;
 
@@ -43,12 +39,6 @@ export const FixedToolbar: FC<FixedToolbarProps> = ({
         onMouseDown={(e) => e.preventDefault()}
       >
         <div className={classes.inner}>
-          {/* {isGenerativeAiEnabled && (
-            <>
-              <AskAiGroup />
-              <div className={classes.divider} />
-            </>
-          )} */}
           <BlockTypeGroup editor={editor} />
           <div className={classes.divider} />
           <InlineMarksGroup editor={editor} state={state} />