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/workspace/types/workspace.types.ts

@@ -20,7 +20,6 @@ export interface IWorkspace {
   plan?: string;
   enforceMfa?: boolean;
   aiSearch?: boolean;
-  generativeAi?: boolean;
   disablePublicSharing?: boolean;
   mcpEnabled?: boolean;
   aiChat?: boolean;
@@ -61,7 +60,6 @@ export interface IWorkspaceApiSettings {
 
 export interface IWorkspaceAiSettings {
   search?: boolean;
-  generative?: boolean;
   mcp?: boolean;
   chat?: boolean;
   dictation?: boolean;