Add South Africa (SAST) timezone option and default; fix UTC-only clocks

[Poid-ZA]

Summary

• `InterceptTime` (the global timezone preference used across the app) only offered UTC / browser-local / US zones, defaulting to `US/Eastern`. Added `Africa/Johannesburg` (labelled SAST) as a selectable option in the settings modal and as the new default.

• Fixed two clocks that bypassed `InterceptTime` entirely and always rendered raw UTC regardless of the user's preference:

  • the per-map HUD clock in `map-utils.js` (top-right overlay on every dashboard map)
  • the GPS fix-time readouts in `gps.js` (sidebar + visuals panel)

  Both now show local time (per `InterceptTime`, SAST by default) with UTC appended for reference, instead of UTC-only.

• `templates/layout/base.html`'s header clock was fixed the same way for consistency, though no current route renders that template.

Non-SAST users are unaffected beyond the new default — anyone who already set a timezone preference in Settings keeps it (stored in `localStorage`); this only changes the out-of-the-box default and adds a previously-missing option.

Test plan

• `node --check` on all three edited JS files
• Manual review of `InterceptTime.onChange` wiring — no route currently renders `base.html`, so that portion is inert but consistent with the rest of the codebase

🤖 Generated with Claude Code

[smittix]

Thanks for this — the clock work is a real improvement and I want to take it.

The header clock, map HUD and GPS fix time all hardcoded UTC, and routing them through InterceptTime with a UTC fallback when it isn't loaded is the right shape. Adding SAST as an option is obviously fine too.

One change before I merge, in static/js/core/utils.js:

-let _timezone = localStorage.getItem('interceptTimezone') || 'US/Eastern';
+let _timezone = localStorage.getItem('interceptTimezone') || 'Africa/Johannesburg';

This changes the default for every user who hasn't explicitly chosen a timezone. US/Eastern being the current default is itself wrong, so I understand the instinct — but swapping it for Africa/Johannesburg trades one arbitrary regional default for another, and it would silently move existing users' clocks to SAST on update.

'local' is already in TZ_MAP (mapped to undefined, so Intl uses the browser's own zone) and is the neutral answer: everyone gets their own time without configuring anything, wherever they are.

Could you change that line to || 'local'? The several comments that mention "default SAST" would want a small update to match. Everything else in the PR is good to go as-is.

[smittix]

Thanks! The SAST timezone option and the UTC-only clock fixes (map HUD clock, GPS time respecting InterceptTime prefs) are genuinely useful and I'd like to take them. One change needed before merge: please don't change the global default timezone — this PR switches the default from US/Eastern to Africa/Johannesburg for all users. Could you keep the existing default (or, better, default to local) and just add SAST as a selectable option? With that adjusted + a rebase onto current main, this is good to go.