feat(control-plane): add i18n for knowledge base and refine mental models

[Sanderhoff-alt]

Problem Description

1. Missing i18n in Knowledge Base:
   • The Knowledge Base (knowledge-base-view.tsx) was untranslated across all non-English locale dictionaries (zh-CN, zh-TW, yue-Hant, ja, ko, es, fr, de, pt), with 32+ keys left as English fallback strings.
2. Terminology & Translation Refinements:
   • Streamlined Chinese terminology for Knowledge Pages to 「知识页」 / 「知識頁」.
   • Refined documentsDescription across all 10 locales to accurately convey the memory extraction/retain pipeline instead of literal retention.
   • Simplified and clarified staleness status tooltips (staleAutoTitle, staleManualTitle, inSyncTitle) and history diff headers (basedOnDiff → 「依据对比」).
3. Hardcoded User-Facing Strings:
   • Two UI strings in mental-models-view.tsx were hardcoded in English instead of using next-intl translation keys:
     • Clear content success toast (toast.success("Mental model content cleared"))
     • Detail view button (<Button>Open</Button>)

Proposed Solution

1. Complete Multilingual Dictionaries for Knowledge Base:
   • Fully translated all knowledgeBase.* keys across all 10 supported locales:
     • en, zh-CN, zh-TW, yue-Hant, ja, ko, es, fr, de, pt.
     • Localized fieldSourceQuery, editSourceQueryHint, howDerived, backedBy, search, and folder/page CRUD dialogs.
2. Refine Mental Models, Documents, and Diagnostics Localizations:
   • Streamlined Knowledge Pages to 知识页 (zh) / 知識頁 (zh-TW, yue).
   • Retained standard repository terminology for Source Query (源查询 / 來源查詢 / ソースクエリ / 소스 쿼리 / etc.) and Delta / Full refresh modes (增量 / 完整).
   • Clarified documentsDescription to accurately express document memory extraction across all 10 locales.
   • Clarified mental model staleness tooltips (staleAutoTitle, staleManualTitle, inSyncTitle) and simplified basedOnDiff to 「依据对比」 / 「依據對比」.
3. Eliminate Hardcoded Strings in Components:
   • Connected toast.success(t("toastContentCleared")) and {t("actionOpenDetail")} in mental-models-view.tsx.
   • Added corresponding entries to all 10 message files.

Verification

• i18n Static Check: Ran npm run i18n:check (tsx scripts/find-untranslated.ts), confirmed ✓ no hardcoded user-facing strings found.
• Unit Tests:
  • Ran npx vitest run tests/messages/ (22/22 tests passed, verifying all static t(...) keys and message dictionaries across locales).
• Pre-commit Hooks: Verified all lint and pre-commit checks pass cleanly.
• Local Dev Server: Verified rendered UI and language switching on local Next.js dev server.