v0.8.59: Bring the website to parity with existing Japanese and Vietnamese README locales

[Hmbown]

Problem

Japanese and Vietnamese already exist as README translations, but the website only ships English and Chinese. That makes the project look partially localized: users can find translated README content on GitHub but cannot browse the website in the same languages.

This should be the first website localization implementation wave because it does not require choosing new community languages. It brings the website up to parity with translated README work that already exists.

Evidence

• README.ja-JP.md exists and is linked from README.md:6.
• README.vi.md exists and is linked from README.md:6.
• web/lib/i18n/config.ts:1 exposes only en and zh.
• web/lib/i18n/dictionaries/ contains only en.ts and zh.ts.
• web/components/locale-switcher.tsx:6-13 is a two-locale toggle and cannot scale to Japanese/Vietnamese without redesign.
• Website pages live under web/app/[locale]/, so the route structure is already locale-ready.

Proposed direction

Add website locales for:

• ja-JP or route alias ja
• vi

Use #3090's matrix decision for exact route naming if that issue has landed first.

Scope should include the main public pages, not only the homepage:

• homepage
• install
• docs
• FAQ
• roadmap/feed/contribute surfaces where currently localized through dictionaries or inline text
• footer/nav/locale selector labels

Acceptance criteria

• Website i18n config includes Japanese and Vietnamese route entries.
• Website dictionaries exist for Japanese and Vietnamese.
• Locale switcher can display and route among en, zh, Japanese, and Vietnamese.
• Accept-Language detection can route ja, ja-JP, vi, and vi-VN requests to the appropriate locale.
• Public pages do not silently fall back to English for visible primary copy unless explicitly marked as untranslated.
• README language links mention the website locale if the README includes website/documentation links.
• Long translated strings fit nav, cards, buttons, and mobile layouts.
• Translation source is synchronized with the post-v0.8.65 end-cap: Rewrite README with CodeWhale history, Fleet, and provider routing map #3087 README/product language or clearly marked as temporarily derived from the current README.

Verification

npm --prefix web run lint
npm --prefix web test -- --run
npm --prefix web run build

Manual checks:

1. Visit /ja or /ja-JP and /vi depending on the matrix decision.
2. Switch among English, Chinese, Japanese, and Vietnamese from the same path.
3. Check mobile and desktop widths for nav/footer/card text overflow.
4. Confirm install commands and provider/model names are not mistranslated.