fix: prevent buttons from resizing during loading state

- Added white-space: nowrap to .admin-btn (prevents text wrapping)
- Modal footer buttons get min-width: 100px for consistent sizing
- Spinner in buttons doesn't add extra vertical space

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/admin/admin.css

@@ -700,6 +700,7 @@ img {
   font-family: inherit;
   cursor: pointer;
   transition: var(--transition);
+  white-space: nowrap;
 }
 
 .admin-btn:disabled {
@@ -707,6 +708,14 @@ img {
   cursor: not-allowed;
 }
 
+/* Prevent buttons from growing when text changes to spinner + loading text */
+.admin-modal-footer .admin-btn {
+  min-width: 100px;
+}
+.admin-modal-footer .admin-btn .admin-spinner {
+  margin: -2px 0;
+}
+
 .admin-btn-sm {
   padding: 6px 11px;
   font-size: 12px;