feat: cover image fallback for home grid cards

- index.php: batch-load thesis_files covers for theses without banner_path
- Resolution order: banner_path → cover file → gradient placeholder
- Uses single IN() query to avoid N+1 problem

public/index.php

@@ -28,6 +28,26 @@ try {
     }
 
     $totalPages = $isDefaultView ? 1 : (int)ceil($totalItems / $itemsPerPage);
+
+    // Batch-load cover images for theses that have no banner_path
+    $coverMap = [];
+    if (!empty($itemsToLoad)) {
+        $needCover = array_column(
+            array_filter($itemsToLoad, fn($t) => empty($t['banner_path'])),
+            'id'
+        );
+        if (!empty($needCover)) {
+            $ph   = implode(',', array_fill(0, count($needCover), '?'));
+            $cStmt = $db->getConnection()->prepare("
+                SELECT thesis_id, file_path FROM thesis_files
+                WHERE file_type = 'cover' AND thesis_id IN ($ph)
+            ");
+            $cStmt->execute($needCover);
+            foreach ($cStmt->fetchAll() as $row) {
+                $coverMap[$row['thesis_id']] = $row['file_path'];
+            }
+        }
+    }
 } catch (Exception $e) {
     error_log("Error loading theses: " . $e->getMessage());
     $itemsToLoad = [];
@@ -36,6 +56,7 @@ try {
     $totalItems  = 0;
     $latestYear  = null;
     $isDefaultView = false;
+    $coverMap = [];
 }
 
 $currentNav = '';
@@ -91,7 +112,11 @@ $currentNav = '';
                                 $thumb = $item['banner_path'];
                             }
 
-                            // 2. Cover image from thesis_files (not returned by view — skip for now)
+                            // 2. Cover image from thesis_files (batch-loaded above)
+                            if (!$thumb && isset($coverMap[$item['id']])) {
+                                $thumb = $coverMap[$item['id']];
+                            }
+
                             // 3. Fall through to gradient
                             ?>
                             <?php if ($thumb): ?>