Add delete/batch-delete and sortable columns to admin list

- Database: add deleteThesis() and bulkDeleteTheses() methods with file cleanup
- Database: add SORT_MAP + buildOrderBy() for safe column sorting
- Database: getThesesList() now respects sort/dir filter params
- New action: actions/delete.php (single + batch delete with CSRF)
- Admin index: delete button per row with confirmation dialog
- Admin index: batch 'Supprimer' button in bulk actions bar
- Admin index: sortable column headers (ID, Titre, Année, Orientation, AP, Statut)
- Admin index: sort state preserved in pagination links
- CSS: admin-btn-delete (red muted), admin-sort-link styles

public/assets/css/admin.css

@@ -498,6 +498,20 @@
     white-space: nowrap;
 }
 
+/* Sortable column headers */
+.admin-sort-link {
+    color: inherit;
+    text-decoration: none;
+    display: inline-flex;
+    align-items: center;
+    gap: 2px;
+    transition: color 0.15s;
+}
+
+.admin-sort-link:hover {
+    color: var(--accent-primary);
+}
+
 .admin-body table td {
     padding: var(--space-2xs) var(--space-xs);
     border-bottom: 1px solid var(--border-primary);
@@ -621,6 +635,15 @@
     background: var(--bg-tertiary);
 }
 
+.admin-btn-delete {
+    background: var(--error-muted-bg);
+    color: var(--error);
+    border-color: var(--error-muted-border, var(--border-primary));
+}
+.admin-btn-delete:hover {
+    filter: brightness(0.9);
+}
+
 .publish-form {
     display: inline;
     margin: 0;