On Distributions, fix line-item column headers wrapping and alignment on narrow screens

Summary

On distributions (both when fulfilling a request and on the regular form), making your screen narrower breaks the line-item column headers: "Quantity - Total Units" wraps onto three lines and the headers stop lining up with the inputs/values below them. Tidy up the header row so it stays readable and aligned on narrow screens.

Why

The form looks broken on narrow screens — the header text wraps awkwardly and drifts out of alignment with its columns, making "Items in this distribution" hard to read.

Details

This was noticed while fixing #5556 (the item dropdown not resizing). With the dropdown now responsive to window width, the remaining roughness is in the header row of "Items in this distribution":

• The "Quantity - Total Units" header wraps onto three lines on a narrow window.
• The "Requested" header (and the "Quantity" header) no longer line up vertically with the input/value they label.

To see this, sign in as org_admin1@example.com, click Requests, click view beside a started request, click "Fulfill request" (this also happens on the regular new distribution form), then make the window narrow.

Criteria for completion

• line-item column headers stay readable (no awkward multi-line wrapping) on narrow screens
• headers line up with the inputs/values they label
• include before/after screenshots in your PR

Reproduce the issue from Requests by opening a started request, selecting “Fulfill request,” and narrowing the window; compare it with the regular new distribution form. Inspect the “Items in this distribution” line-item header row and verify that the Quantity, Total Units, and Requested headers remain readable and aligned with their inputs or values, then capture before-and-after screenshots.