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/admin/add.php

@@ -107,7 +107,7 @@ function wasSelected($key, $value) {
             </div>
 
             <!-- Lecteur·ices (dynamic) -->
-            <div class="admin-form-row" style="align-items:start;">
+            <div class="admin-form-row">
                 <label class="admin-label">Lecteur·ices :</label>
                 <div id="jury-lecteurs-list" class="admin-jury-list">
                     <!-- rows injected by JS; start with one empty row -->
@@ -119,7 +119,7 @@ function wasSelected($key, $value) {
                         <button type="button" class="admin-btn-remove" onclick="removeJuryRow(this)" aria-label="Supprimer ce lecteur">✕</button>
                     </div>
                 </div>
-                <button type="button" class="admin-btn-secondary" style="margin-top:.5rem;"
+                <button type="button" class="admin-btn-secondary admin-add-jury-btn"
                         onclick="addJuryRow()">+ Ajouter un·e lecteur·ice</button>
             </div>
         </fieldset>
@@ -235,7 +235,7 @@ function wasSelected($key, $value) {
         </div>
 
         <!-- Synopsis -->
-        <div class="admin-form-row" style="align-items:start;">
+        <div class="admin-form-row">
             <label class="admin-label" for="synopsis">Synopsis :</label>
             <textarea class="admin-textarea" id="synopsis" name="synopsis"
                       rows="7" required><?= old('synopsis') ?></textarea>
@@ -275,7 +275,7 @@ function wasSelected($key, $value) {
         </div>
 
         <!-- Image couverture -->
-        <div class="admin-form-row" style="align-items:start;">
+        <div class="admin-form-row">
             <label class="admin-label">Image de couverture :</label>
             <div class="admin-file-input">
                 <input type="file" id="couverture" name="couverture" accept="image/jpeg,image/png">
@@ -284,7 +284,7 @@ function wasSelected($key, $value) {
         </div>
 
         <!-- Image bannière -->
-        <div class="admin-form-row" style="align-items:start;">
+        <div class="admin-form-row">
             <label class="admin-label">Image bannière (accueil) :</label>
             <div class="admin-file-input">
                 <input type="file" id="banner" name="banner" accept="image/jpeg,image/png,image/webp">
@@ -293,7 +293,7 @@ function wasSelected($key, $value) {
         </div>
 
         <!-- Fichiers -->
-        <div class="admin-form-row" style="align-items:start;">
+        <div class="admin-form-row">
             <label class="admin-label">Fichiers du TFE :</label>
             <div class="admin-file-input">
                 <input type="file" id="files" name="files[]" multiple