fix: use Files.copy for full-file responses (proven working), FileChannel.transferTo only for Range requests

src/main/java/com/dostalgia/StaticResource.java

@@ -159,14 +159,18 @@ public class StaticResource {
 
     /**
      * Streaming file output that avoids loading the whole file into memory.
-     * Supports partial reads (offset + length) for HTTP Range requests.
+     * Uses Files.copy for full-file (zero-copy via sendfile) and a position-limited
-     * Uses FileChannel.transferTo() for zero-copy sendfile semantics.
+     * transfer for HTTP Range requests.
      */
     private record FileStream(Path file, long offset, long length) implements StreamingOutput {
         @Override
         public void write(OutputStream output) throws IOException {
+            if (offset == 0 && length == Files.size(file)) {
+                // Full file — use simple Files.copy (proven working)
+                Files.copy(file, output);
+            } else {
+                // Partial content (Range request) — transfer only the requested bytes
                 try (FileChannel ch = FileChannel.open(file, StandardOpenOption.READ)) {
-                if (length > 0) {
                     ch.transferTo(offset, length, java.nio.channels.Channels.newChannel(output));
                 }
             }