feat(web): extensiones por cliente (campos estándar adicionales y U_*) por módulo, y campo de selección

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

apps/web/src/app/features/documents/document-form.page.html

@@ -111,6 +111,17 @@
         (valueChange)="setHeader(f.key, $event)"
       />
     }
+    @case ('select') {
+      <sbo-select-field
+        [label]="f.label"
+        [required]="!!f.required"
+        [readonly]="readOnly()"
+        [options]="f.options"
+        placeholder=""
+        [value]="str(header()[f.key]) || null"
+        (valueChange)="setHeader(f.key, $event ?? '')"
+      />
+    }
     @case ('number') {
       <sbo-number-field
         [label]="f.label"