Bug: Footer.of_children omits width=100% on its outer table

[sabine]

Footer.of_children (added in #10) emits:

<table border="0" cellpadding="0" cellspacing="0" role="presentation">
  <tr>
    <td>[children]</td>
  </tr>
</table>

Every other structural component in typemail (Section, Callout, Spacer) adds width="100%" to its outer <table>. Without it, the footer table collapses to the content width in Outlook and right-aligns or left-aligns unpredictably depending on surrounding padding. The result is a misaligned footer block that doesn't sit flush with the rest of the email body.

Repro

let footer =
  Footer.of_children
    [Paragraph.to_element (Paragraph.v "Line 1");
     Paragraph.to_element (Paragraph.v "Line 2")]
  |> Footer.to_element

Rendered (whitespace-stripped):

<table border="0" cellpadding="0" cellspacing="0" role="presentation">
  <tr>
    <td>
      <p>Line 1</p>
      <p>Line 2</p>
    </td>
  </tr>
</table>

No width="100%". Compare to Section.v [...] which does include it. The string-content Footer.v path is probably the same (worth checking).

Fix

lib/footer.ml — add "width", "100%" to the base attribute list for both v and of_children rendering paths. Update the structure test to assert width="100%" on the footer's outer table, mirroring the Section test.

Filed during feedmansion.com#1024.