Fix table jitter, horizontal scroll, and vertical scroll snapping on /lignes

Root cause: the main table used auto layout, so every loadOlder()/
loadNewer() reflowed every column's width based on whatever was
currently loaded -- visibly shifting the table, spilling past the
viewport into a horizontal scrollbar, and corrupting loadOlder()'s
scroll-position compensation (which assumes the scrollHeight delta
after prepending rows is *only* the new rows' own height -- not true
once existing rows also reflow). That's what made scrolling up feel
like it kept snapping back down.

- table-layout: fixed with explicit per-column widths (percentages
  throughout, not mixed with rem -- mixing meant the rem columns' width
  was added on top of the percentage budget instead of coming out of
  it), plus box-sizing: border-box so padding doesn't inflate columns
  beyond their declared width.
- overflow-x: hidden instead of auto on the scroll container: with both
  x and y auto on the same element, the browser has to guess whether a
  vertical scrollbar will appear before laying out width: 100%, and a
  wrong guess understates available width by a scrollbar's worth --
  exactly enough to tip a tightly-fitting table into needing horizontal
  scroll too.
- Fixed a footer-row column count bug found along the way: it still had
  two actions-col cells and a colspan=5 label from before the link
  button was removed, leaving the label 6 columns short of Signalement
  and misaligning every footer cell after it.

Verified with a 25-round scripted scroll-up stress test spanning 3
loadOlder() triggers: scroll position stays correctly anchored near
where the user was looking, never jumps toward the bottom.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/modules/custom/figli_compta_ledger/templates/figli-compta-home.html.twig

@@ -250,8 +250,7 @@
       <tfoot>
         <tr class="figli-totals-row">
           <td class="actions-col"></td>
-          <td class="actions-col"></td>
-          <td colspan="5">
+          <td colspan="6">
             Solde {{ currentYear || '…' }} (créditeur / débiteur)
             <span v-if="currentYearLoading" class="figli-note">chargement…</span>
           </td>