[Feat] Keynote: full 8-language translations and dark/light toggle

[samzong]

Summary

Add complete translations for all 8 supported languages (en, zh, ja, ko, fr, de, es, pt) across the entire keynote presentation, and restore the dark/light theme toggle that was previously disabled.

Type of change

• [Feat] new feature

Why is this needed?

The keynote language switcher advertised 8 languages but only en and zh had actual translations — selecting ja/ko/fr/de/es/pt all fell back to English. Additionally, the colorSchema: dark setting locked the presentation to dark mode only, removing the previously working theme toggle.

What changed?

• Added ja, ko, fr, de, es, pt translations to every I18nText object across slides.md and all 16 Vue components (~200 translation keys, ~1200 new translation values)
• Changed colorSchema from dark to all in slides.md frontmatter to re-enable Slidev built-in dark/light toggle
• Replaced French ASCII apostrophes (') with Unicode U+2019 ('’) to fix Slidev title-renderer Vue compilation errors (e.g. Qualité d'ingénierie broke template parsing)

Architecture impact

• Owning layer: keynote (standalone Slidev package, no cross-layer impact)
• Cross-layer impact: none
• Invariants touched from docs/architecture-invariants.md: none
• Why those invariants remain protected: keynote is an independent package with no imports from desktop/shared

Linked issues

N/A

Validation

• pnpm dev (keynote) — all 8 languages render correctly
• Dark/light toggle works via Slidev controls
• No Slidev compilation errors after apostrophe fix
• pnpm lint
• pnpm test
• pnpm build
• pnpm check:ui-contract
• Manual smoke test
• Not run (keynote-only change, no desktop code affected)

Screenshots or recordings

N/A — keynote presentation changes only, no desktop UI affected.

Release note

• No user-facing change. Release note is NONE.

NONE

Checklist

• The PR title uses at least one approved prefix: [Feat]
• The summary explains both what changed and why
• Validation reflects the commands actually run for this PR
• Architecture impact is described and references any touched invariants
• Cross-layer changes are explicitly justified
• The release note block is accurate

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