Refactor: Clean up batch header reading in server

Replace manual unpacking of batch_meta_header_t fields with direct
struct field reads. The magic was already read separately, so we
now read count, total_name_len, and total_size directly into the
struct fields, which is cleaner and less error-prone.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

src/server.c

@@ -303,19 +303,15 @@ void handle_client(int sock_fd, const char *dest_dir) {
 
         if (magic == MAGIC_BATCH_META) {
             // Handle batch metadata
-            // We already read the magic, now read the rest of the header
+            // We already read the magic (4 bytes), now read the rest of the header (16 bytes)
             batch_meta_header_t batch_hdr;
-            uint32_t rest_of_header[3]; // count, total_name_len, total_size_hi
-            uint32_t total_size_lo;
-            if (readn(sock_fd, &rest_of_header, sizeof(rest_of_header)) != sizeof(rest_of_header) ||
-                readn(sock_fd, &total_size_lo, sizeof(total_size_lo)) != sizeof(total_size_lo)) {
+            batch_hdr.magic = magic;
+            if (readn(sock_fd, &batch_hdr.count, sizeof(batch_hdr.count)) != sizeof(batch_hdr.count) ||
+                readn(sock_fd, &batch_hdr.total_name_len, sizeof(batch_hdr.total_name_len)) != sizeof(batch_hdr.total_name_len) ||
+                readn(sock_fd, &batch_hdr.total_size, sizeof(batch_hdr.total_size)) != sizeof(batch_hdr.total_size)) {
                 fprintf(stderr, "Failed to read batch header\n");
                 break;
             }
-            batch_hdr.magic = magic;
-            batch_hdr.count = rest_of_header[0];
-            batch_hdr.total_name_len = rest_of_header[1];
-            batch_hdr.total_size = ((uint64_t)rest_of_header[2] << 32) | total_size_lo;
 
             if (batch_hdr.count > BATCH_MAX_FILES) {
                 fprintf(stderr, "Batch count %u exceeds max %d\n", batch_hdr.count, BATCH_MAX_FILES);