Trim implementation-detail comments in magic_link.html.heex

[mdon]

Summary

Codex final-review CONCERN from PR #559: the long inline comments
explaining the data-phx-skip / 183px / form-ownership root causes
in magic_link.html.heex belong in the PR description, not in
templates where they'll age out faster than the fix.

Kept short comments naming the root rules ("fieldset default
min-width", "double-swap broke the DOM merge") so a future edit
sees the intent without the wall of text.

Notes

• HTML-template-only change. No logic touched.
• Surfaced during PR Fix submit button overflow on auth form cards (fieldset min-width) #559 review; landed late and got orphaned
  on the merged branch. This PR re-applies it cleanly off
  upstream/dev.
• The fix in PR Fix submit button overflow on auth form cards (fieldset min-width) #559 (min-w-0 on <fieldset> + removed
  phx-disable-with to avoid double-swap DOM conflict) remains
  in place; only the explanatory comments are trimmed.

Test plan

• mix compile clean
• mix format --check-formatted clean on the touched file
• Visual check at /phoenix_kit/users/log-in/magic-link —
  submit button still fills the card, no overflow

[mdon]

Superseded by #565 — consolidated translation follow-ups into one PR per module per @mdon convention. The same commits are cherry-picked onto the new branch verbatim.