Remove inline styles from admin templates; extract to admin.css utility classes

- login.php: removed style= on .admin-form-row and .admin-label (already covered
  by .admin-login-box scoped rules); extracted submit-wrap spacing and full-width
  button to .admin-login-box .admin-submit-wrap and .admin-login-box .admin-btn
- account.php: style="margin-top:3rem" on danger-zone heading moved to
  .admin-section-title--danger modifier; <span style="color:..."> replaced with
  <small> element styled via .admin-danger-zone__description small
- add.php / edit.php / pages-edit.php: all style="align-items:start" removed from
  .admin-form-row (redundant — already the CSS default at line 116 of admin.css);
  banner preview inline styles extracted to .admin-banner-preview / .admin-banner-preview img;
  add-jury button margin extracted to .admin-add-jury-btn; cancel links use .admin-cancel-link

Zero inline style= attributes remain in login, account, add, edit, pages-edit.

public/assets/css/admin.css

@@ -604,6 +604,15 @@
   margin-bottom: 0.2rem;
 }
 
+.admin-login-box .admin-submit-wrap {
+  margin-top: 1rem;
+  padding-top: 0.5rem;
+}
+
+.admin-login-box .admin-btn {
+  width: 100%;
+}
+
 /* Import page */
 .admin-import-area {
   display: flex;
@@ -835,6 +844,43 @@
   line-height: 1.5;
 }
 
+.admin-danger-zone__description small {
+  color: var(--admin-text-muted);
+  font-size: 0.9rem;
+}
+
+.admin-section-title--danger {
+  margin-top: 3rem;
+}
+
+/* Banner image preview in edit form */
+.admin-banner-preview {
+  margin-bottom: 0.5rem;
+}
+
+.admin-banner-preview img {
+  max-width: 320px;
+  max-height: 100px;
+  object-fit: cover;
+  border: 1px solid #444;
+  display: block;
+}
+
+.admin-banner-preview .admin-checkbox-label {
+  margin-top: 0.35rem;
+  display: block;
+}
+
+/* Cancel / secondary action link in submit row */
+.admin-cancel-link {
+  margin-left: 0.75rem;
+}
+
+/* Add-jury button below list */
+.admin-add-jury-btn {
+  margin-top: 0.5rem;
+}
+
 /* ============================================================
    ACCESSIBILITY UTILITIES
    ============================================================ */