feat(website): show limited-time free promo label

[ppapsmoken]

Problem

The welcome promotion card previously split the free offer into separate labels, which made the right-side badge inconsistent across locales and cramped the model details.

Change

• Combine the first model offer into a localized limited-time-free label (for example, 限时免费 in Chinese).
• Keep the offer column aligned on the far right and allow model descriptions to wrap instead of being truncated on desktop.
• Add coverage for the new localized copy across all website locales.

Validation

• bun test src/components/welcome-promo-modal.test.ts
• bun run typecheck
• bun run lint (existing img-element warnings only)
• bun run build
• Checked homepage modal on /, /zh, /es, /fr, /pt, /ru, /ja, /vi, /de, /id.

Impact / risk

Website-only UI and copy change. No router, console, database, or API relay behavior changes.