Resilient flatten: catch exceptions + return meaningful error messages

- Wrap flattenSingleDir in try-catch — if it fails, the upload continues
  and the cd <subdir> fix in dosbox.conf handles subdirectory executables
- Add global catch to upload method returning the actual exception
  message instead of a generic 500
- This will help diagnose why the user's King's Quest VI zip fails:
  flattening can throw on special filenames, Mac resource forks,
  symlinks, or other unusual ZIP contents

src/main/java/com/dostalgia/UploadResource.java

@@ -64,7 +64,12 @@ public class UploadResource {
             Path extractDir = tmpDir.resolve("extracted");
             unzip(zipPath, extractDir);
             // If the ZIP has a single root directory, flatten it
-            flattenSingleDir(extractDir);
+            try {
+                flattenSingleDir(extractDir);
+            } catch (Exception e) {
+                LOG.warning("Flatten failed for '" + filename + "': " + e.getMessage()
+                    + " — continuing (cd in autoexec handles subdirs)");
+            }
 
             // Find main executable
             String mainExe = findMainExe(extractDir);
@@ -120,6 +125,11 @@ public class UploadResource {
 
             return Response.status(201).entity(game).build();
 
+        } catch (Exception e) {
+            LOG.severe("Upload failed for '" + filename + "': " + e.getMessage());
+            return Response.serverError()
+                .entity(Map.of("error", "Upload failed: " + e.getMessage()))
+                .build();
         } finally {
             deleteDir(tmpDir);
         }