Tech: update CSS for new attachment components

Add CSS classes for FileFieldComponent:
- .attachment-field for common styles
- .attachment-field--single for max=1 cases
- .attachment-field--multiple for max>1 cases

Keep existing .attachment-drop-zone styles (reused by DropZoneDecorator).
Maintain legacy styles for backward compatibility during transition.

Part of attachment components refactoring (#6/7)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: mfo

app/assets/stylesheets/attachment.scss

@@ -21,6 +21,46 @@
   }
 }
 
+// New unified attachment field styles
+.attachment-field {
+  // Common styles for all attachment fields
+
+  // Anti-virus badge for just uploaded attachment when seen as downloadable
+  .fr-download + .fr-badge {
+    position: relative;
+    top: -1rem;
+  }
+
+  .fr-btn {
+    align-self: flex-start;
+  }
+
+  &.attachment-field--single {
+    // Styles for single file (max=1)
+  }
+
+  &.attachment-field--multiple {
+    // Styles for multiple files (max>1)
+    &:not(.fr-downloads-group),
+    &.fr-downloads-group[data-controller='replace-attachment'] {
+      ul {
+        list-style-type: none;
+        padding-inline-start: 0;
+      }
+
+      li {
+        padding-bottom: 0;
+      }
+    }
+
+    &.fr-downloads-group.destroyable ul {
+      list-style-type: none;
+      padding-inline-start: 0;
+    }
+  }
+}
+
+// Legacy support for old class names (will be removed)
 .attachment-multiple:not(.fr-downloads-group),
 .attachment-multiple.fr-downloads-group[data-controller='replace-attachment'] {
   ul {
@@ -50,6 +90,7 @@
   }
 }
 
+// Legacy support
 .attachment-multiple.fr-downloads-group.destroyable ul,
 ul[data-file-input-reset-target='fileList'] {
   list-style-type: none;