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/public/locales/ja-JP/translation.json

@@ -665,9 +665,6 @@
   "AI-powered search (AI Answers)": "AI搭載検索 (AI回答)",
   "AI search uses vector embeddings to provide semantic search capabilities across your workspace content.": "AI検索はベクター埋め込みを使用してワークスペース全体の意味検索を実現します",
   "Toggle AI search": "AI検索を切り替え",
-  "Generative AI (Ask AI)": "生成AI (Ask AI)",
-  "Enable AI-powered content generation in the editor. Allows users to generate, improve, translate and transform text.": "エディターでAIを活用したコンテンツ生成を有効にします。ユーザーがテキストの生成、改善、翻訳、および変換を行うことができます。",
-  "Toggle generative AI": "生成AIを切り替える",
   "Upgrade your plan": "プランをアップグレードする",
   "Available with a paid license": "有料ライセンスで利用可能",
   "Upgrade your license tier.": "ライセンスタイアをアップグレードしてください。",