Replace the comma-separated tags field with one field per linked entrée

field_entree_liee is multi-value, but the single #tags entity_autocomplete
rendered it as one comma-separated text box -- functionally correct but
easy to mistake for "only one entrée can be linked". Rebuilt as one
autocomplete field per link plus an "Ajouter une référence" button (the
classic Drupal multi-value pattern, matching the Répartition paragraphs
widget already used elsewhere in this form). Clearing a field's text
drops that link on save, rather than a "remove" button -- a remove
button would need to shift every later delta, which fights Drupal's own
value-restoration-on-AJAX-rebuild (verified live: that restoration only
works cleanly for pure appends at a stable delta, which is all
addItem() does).

Verified end-to-end via raw AJAX POSTs against a real 3-entrée versement:
initial load pre-fills 3 separate fields, "Ajouter" appends a 4th empty
one without disturbing the first 3, and submitting with one field
cleared and a new one filled saves exactly the expected set.

web/modules/custom/figli_compta_ledger/css/admin-chrome.css

@@ -36,6 +36,13 @@ html.gin--dark-mode .figli-page-nav a.is-active {
   color: #e8e9ea;
 }
 
+/* "Lier à une entrée client" modal (LinkEntreeForm): one autocomplete
+   field per linked entrée instead of a single comma-separated field --
+   tighten the default spacing between them a little. */
+#figli-entree-liee-items .form-item {
+  margin-bottom: 0.5rem;
+}
+
 /* Drupal status messages: fixed top-right, out of document flow, so they
    never shift the page layout the way the default in-flow placement does
    -- and shrunk to a fraction of Gin's default size, which is sized for a