[FEATURE]: Add Vietnamese (vi) locale to the Console #55927

Vietnamese has been available in the desktop and web app for a while (packages/app/src/runtime/i18n/vi.ts, packages/ui/src/i18n/vi.ts), but the Console marketing site has no Vietnamese locale at all.

Vietnamese visitors of opencode.ai currently get the English pages, the English pricing/FAQ/enterprise copy, and English docs links, and Accept-Language: vi or vi-VN is ignored.

Adding vi to the Console means:

• a Vietnamese dictionary under packages/console/app/src/i18n
• vi in LOCALES, the picker label, the vi-VN html tag, and Accept-Language matching in packages/console/app/src/lib/language.ts
• the /vi/... routes render, and <html lang> is vi-VN

Terminology can follow what packages/app/src/runtime/i18n/vi.ts already uses (tác nhân, phiên, tệp, nhà cung cấp, mô hình) so the Console and the app read as one product.