fix: printed header uses US date/time format rather than the device locale

[mohitsatr]

Fixes #11109

[github-actions]

✅ Validation Passed: All report and feature-flag labels are correctly set.

[AvikMakwana]

Thanks for working on this.

I had also investigated #11109 locally and reached the same production fix: replacing the hardcoded M/d/yyyy, h:mm a print-header formatter with Android’s locale-aware DateFormat.getDateFormat(context) and DateFormat.getTimeFormat(context).

I manually verified the behavior on a Motorola Edge 60 Fusion with 24-hour time enabled, using the same email in both builds:

• Thunderbird Beta 20.0b1: Date: 6/11/26, 4:31 PM
• Patched local build: Date: 11/06/26, 16:31

So this confirms the fix respects local date ordering and the 24-hour time preference.

I also have a small Robolectric regression test for a German region / 24-hour setup. Happy to share that patch if test coverage would be useful here.