[FEATURE]: Timestamps honor LANG/LC_TIME (24h) or add locale/format setting

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Note: This feature request was generated by an LLM—I was just looking for a way to display timestamps in 24-hour format within the TUI. :)

TUI/CLI timestamps always render 12h AM/PM (e.g. 4:11 PM in opencode session list, message timestamps), regardless of system locale. The UI locale is resolved from stored preference → navigator detection → default en; LANG/LC_TIME (here de_DE.UTF-8) are ignored. There is no config option, keybind, or command to change it.

Request either:

1. Resolve the UI locale from LANG/LC_TIME (fallback en), or
2. Add a timestamp format setting (e.g. tui.json timestamp_format / timezone).

Impact: users on 24h locales (de, fr, da, …) get timestamps inconsistent with their system clock. Host-default paths (formatTranscript, toLocaleString()) already honor the env, so this only affects the UI locale.

Context: Intl.DateTimeFormat in packages/session-ui/src/components/message-part.tsx uses i18n.locale(); session list times use Locale.todayTimeOrDateTime; LanguageProvider (packages/app/src/context/language.tsx) resolves locale as localStorage opencode.global.dat:language → navigator.languages → en.

Environment:

• opencode 1.18.18 (TUI/CLI, not web/desktop)
• OS: CachyOS Linux (Arch-based), kernel locale LANG=de_DE.UTF-8 (generated)
• Reproduced via opencode session list and TUI message timestamps

Related: #30347 (timestamps use UTC instead of local time, closed as not
planned) — distinct from this request: that one concerns the timezone
(UTC vs local), this one the clock format (12h vs 24h) driven by the UI
locale.