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/server/src/database/repos/workspace/workspace.repo.ts

@@ -246,7 +246,7 @@ export class WorkspaceRepo {
    * otherwise re-serialize a `JSON.stringify`'d string, yielding a jsonb string
    * that `||` turns into an array). A `jsonb_typeof = 'object'` CASE self-heals
    * workspaces whose `settings.ai.provider` was previously corrupted into an
-   * array/string. Sibling `settings.ai.*` keys (search / generative / chat / mcp
+   * array/string. Sibling `settings.ai.*` keys (search / chat / mcp
    * / systemPrompt) and provider fields absent from the partial are preserved via
    * jsonb `||` merge.
    */