Extract last 3 inline styles from admin templates into CSS classes

admin/thanks.php:
- <div style="margin-top:1.5rem;display:flex;gap:.75rem;flex-wrap:wrap;"> → class="admin-action-bar"
- <p style="color:var(--text-secondary);"> → class="admin-muted"

admin/pages.php:
- Éditer button style="font-size:.8rem;padding:.3rem .75rem;" → class="admin-btn admin-btn--sm"

admin.css (Thesis info sections block):
- Added .admin-action-bar { margin-top:1.5rem; display:flex; gap:0.75rem; flex-wrap:wrap }
- Added .admin-muted { color: var(--text-secondary) }

The only remaining inline style in any admin PHP file is the dynamic
--disk-pct/--disk-color custom properties on the disk bar in system.php,
which carry PHP runtime values and cannot be moved to static CSS.

public/admin/pages.php

@@ -42,7 +42,7 @@ try {
                 <td><?= htmlspecialchars($p['updated_at'] ?? '—') ?></td>
                 <td>
                     <a href="/admin/pages-edit.php?slug=<?= urlencode($p['slug']) ?>"
-                       class="admin-btn" style="font-size:.8rem;padding:.3rem .75rem;">Éditer</a>
+                       class="admin-btn admin-btn--sm">Éditer</a>
                 </td>
             </tr>
             <?php endforeach; ?>