Connectivity test: add a Vietnam country list

Same idea as the other country lists — locals know best. id: 'vietnam', emoji , ≥10 members mixing global services relevant locally with major Vietnamese platforms (e.g. Zalo, VNExpress, Tiki, Shopee, MoMo, VTV…), names in all four locale packs, favicons auto-fetched by pnpm test and committed (full steps in #395).

pnpm check green; comment to claim; PR targets dev.