Add timezone selector (user / UTC) to Flight History page

Problem:

The Flight History page currently shows timestamps only in the user's local timezone (e.g. "START TIME (NZST)"). Operators cross-referencing flight logs, aircraft journey logs, or regulator records — typically in UTC — have to convert times in their heads, which is slow and error-prone.

Desired outcome:

Operators want to switch the Flight History page between their local timezone and UTC, depending on the task they're doing.

Proposed change:

Add a timezone selector on the Flight History page — a dropdown (e.g. NZST / UTC), not a binary toggle — matching the existing selector pattern already used on Flight Explorer. The selector should drive the displayed timestamps and the column header label (e.g. "START TIME (NZST)" → "START TIME (UTC)").