Add signalement (flag) tags for problem lines on /lignes

New field_flag: free-tagging taxonomy reference (multi-value,
auto-create) on ligne_comptable, for marking a line with an
unstructured problem description (e.g. "client impayé") that can't be
detected automatically the way the répartition écart already is.

- New "Signalement" column, inline-editable like Client/Facture/Libellé
  (comma-separated tags, datalist autocomplete, server-side auto-create
  of unknown tags -- same pattern LedgerActionsController already used
  for Client, now shared via findOrCreateTerm()).
- New "Signalées uniquement" filter, mirroring "Écarts uniquement".
- Flagged rows get a distinct amber left-edge accent (box-shadow, not
  border) so a row that's both in écart and signalée shows both
  indicators without one overwriting the other.
- Native node add/edit form gets the field for free via core's
  entity_reference_autocomplete_tags widget.

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

config/sync/field.storage.node.field_flag.yml

@@ -0,0 +1,20 @@
+uuid: 0921a265-d874-487c-bf0a-09a389322d6b
+langcode: en
+status: true
+dependencies:
+  module:
+    - node
+    - taxonomy
+id: node.field_flag
+field_name: field_flag
+entity_type: node
+type: entity_reference
+settings:
+  target_type: taxonomy_term
+module: core
+locked: false
+cardinality: -1
+translatable: true
+indexes: {  }
+persist_with_no_fields: false
+custom_storage: false