Add Locale-Aware Date/Time Formatting for TTL Countdown and Vault History

[maugauwi-hash]

Area: Localization & Internationalization
Priority: Low
Estimated Time: 1.5 hours

Description:
Once vault activity history exists (#216) and the live countdown ships (#220), both need
locale-aware date/time formatting (12h vs. 24h clock, date ordering) rather than a single hardcoded
format, consistent with the currency-formatting concern in #312.

Tasks:

• Use each platform's locale-aware date/time formatting APIs (DateFormatter/java.time formatters)
  rather than fixed format strings
• Add tests for at least one 24-hour-clock locale and one 12-hour-clock locale
• Apply consistently across TTL countdown, history, and notification timestamps