Left-align and restack the fields on the New issue form

Problem

Fields on the New issue page aren't aligned consistently. The Due date box sits in an awkward, misaligned position, and Assignee doesn't line up with Reporter.

Expected behavior

All input fields are left-aligned in one consistent column. Assignee sits directly below Reporter, and Due date lines up with the other fields instead of floating on its own.

Acceptance criteria

• Every field's label and input share the same left edge.
• Assignee is placed directly below Reporter.
• Due date follows the same left edge and width rules as the other fields.
• Vertical spacing between fields is even, and tab order matches the visual order.
• Any other form that shares these fields (e.g. sub-issue creation) gets the same layout.
• Works at the supported responsive breakpoints and in light and dark modes.
• Description placeholder text is tracked separately in TRACK-71.