feat: add cover image field to album creation and editing

- Added dedicated cover image upload field in create_album.php form
- Display current cover image preview when editing
- Drag-and-drop support for cover image with real-time preview
- Shows filename and file size after selection
- Updated save_album.php to handle cover image upload
- Updated update_album.php to handle cover image replacement
- Deletes old cover image when updating
- Cover image optional - first photo in album used as fallback
- Recommended cover dimensions: 500x500px or larger (square)
- File validation: max 5MB, supports JPG, PNG, GIF, WEBP
- All cover image changes included in transaction with rollback on error

src/processors/save_album.php

@@ -58,6 +58,43 @@ try {
         }
     }
 
+    // Handle cover image upload
+    $coverImagePath = null;
+    if (isset($_FILES['cover_image']) && $_FILES['cover_image']['error'] !== UPLOAD_ERR_NO_FILE) {
+        $allowedMimes = ['image/jpeg', 'image/png', 'image/gif', 'image/webp'];
+        $maxSize = 5 * 1024 * 1024; // 5MB
+
+        $fileName = $_FILES['cover_image']['name'];
+        $fileTmpName = $_FILES['cover_image']['tmp_name'];
+        $fileSize = $_FILES['cover_image']['size'];
+        $fileMime = mime_content_type($fileTmpName);
+
+        // Validate file
+        if (!in_array($fileMime, $allowedMimes)) {
+            throw new Exception('Invalid cover image file type');
+        }
+
+        if ($fileSize > $maxSize) {
+            throw new Exception('Cover image file too large (max 5MB)');
+        }
+
+        // Generate unique filename
+        $ext = pathinfo($fileName, PATHINFO_EXTENSION);
+        $newFileName = 'cover_' . uniqid() . '.' . $ext;
+        $filePath = $albumDir . '/' . $newFileName;
+        $coverImagePath = '/assets/uploads/gallery/' . $album_id . '/' . $newFileName;
+
+        if (!move_uploaded_file($fileTmpName, $filePath)) {
+            throw new Exception('Failed to upload cover image');
+        }
+
+        // Update cover image in album record
+        $updateCover = $conn->prepare("UPDATE photo_albums SET cover_image = ? WHERE album_id = ?");
+        $updateCover->bind_param("si", $coverImagePath, $album_id);
+        $updateCover->execute();
+        $updateCover->close();
+    }
+
     // Handle photo uploads
     if (isset($_FILES['photos']) && $_FILES['photos']['error'][0] !== UPLOAD_ERR_NO_FILE) {
         $allowedMimes = ['image/jpeg', 'image/png', 'image/gif', 'image/webp'];
@@ -95,8 +132,8 @@ try {
                 throw new Exception('Failed to upload: ' . $fileName);
             }
 
-            // Set first photo as cover
-            if ($firstPhoto) {
+            // Set first photo as cover if no cover image was uploaded
+            if ($firstPhoto && !$coverImagePath) {
                 $updateCover = $conn->prepare("UPDATE photo_albums SET cover_image = ? WHERE album_id = ?");
                 $updateCover->bind_param("si", $relativePath, $album_id);
                 $updateCover->execute();