Prefer DOS executables over Windows in findMainExe

When a ZIP contains both a DOS version (FALLOUT.EXE) and a Windows
version (FALLOUTW.EXE), the sort now prefers DOS executables.
This reads the EXE header during candidate collection and ranks:
  non-installer → DOS platform → depth → size

Fixes Fallout being mis-detected as a Windows game.

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

@@ -235,7 +235,7 @@ public class UploadResource {
     // ─── Executable Detection ────────────────────────────────────
 
     private String findMainExe(Path dir) throws IOException {
-        record Candidate(int depth, boolean isInstaller, long size, String path) {}
+        record Candidate(int depth, boolean isInstaller, long size, String path, String platform) {}
         List<Candidate> candidates = new ArrayList<>();
 
         Files.walkFileTree(dir, new SimpleFileVisitor<>() {
@@ -253,17 +253,25 @@ public class UploadResource {
 
                 boolean installer = name.contains("install") || name.contains("setup") || name.contains("config");
                 int depth = f.getNameCount() - dir.getNameCount();
-                candidates.add(new Candidate(depth, installer, a.size(), f.toString()));
+                String platform = detectPlatform(f);
+                candidates.add(new Candidate(depth, installer, a.size(), f.toString(), platform));
                 return FileVisitResult.CONTINUE;
             }
         });
 
         if (candidates.isEmpty()) return null;
 
+        // Prefer: non-installer → DOS platform → shallow depth → larger file
         candidates.sort(Comparator
-            .comparingInt((Candidate c) -> c.isInstaller() ? 1 : 0) // non-installer first
+            .comparingInt((Candidate c) -> c.isInstaller() ? 1 : 0)
+            .thenComparingInt(c -> {
+                // Prefer DOS over unknown over Windows
+                if ("dos".equals(c.platform())) return 0;
+                if (c.platform() == null) return 1;
+                return 2; // "windows"
+            })
             .thenComparingInt(Candidate::depth)
-            .thenComparingLong(c -> -c.size())); // larger files first
+            .thenComparingLong(c -> -c.size()));
 
         return candidates.getFirst().path();
     }