feat: support CD-only games (no executable on filesystem)

When no .exe/.com/.bat files are found but CD images are present,
the upload no longer fails. Instead it creates a CD-only game where
the autoexec mounts the CD-ROM image(s) and presents a DOS prompt
with instructions for the user.

- findMainExe returning null + CD images exist = CD-only game
- createBundle handles null exePath (generates CD-only config)
- New buildCdOnlyDosboxConf / buildCdOnlyJsdosJson methods
- Upload metadata handles null executable gracefully (empty string)
- Platform defaults to 'dos' when no executable to inspect

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

@@ -83,10 +83,15 @@ public class UploadResource {
             // Find main executable
             String mainExe = findMainExe(extractDir);
             if (mainExe == null) {
+                // No executable on the filesystem — check if CD images exist instead
+                List<String> cdOnlyImages = findCdImages(extractDir);
+                if (cdOnlyImages.isEmpty()) {
                     return Response.status(400)
                         .entity(Map.of("error", "No executable (.exe/.com/.bat) found in the archive"))
                         .build();
                 }
+                // CD-only game (executable is on the CD image) — proceed without one
+            }
 
             // Find setup executable (SETUP.EXE, INSTALL.EXE, CONFIG.EXE, etc.)
             String setupExe = findSetupExe(extractDir);
@@ -101,7 +106,7 @@ public class UploadResource {
             createBundle(extractDir, mainExe, bundlePath);
 
             // Detect platform (DOS vs Windows) by reading the main EXE header
-            String platform = detectPlatform(Path.of(mainExe));
+            String platform = mainExe != null ? detectPlatform(Path.of(mainExe)) : "dos";
 
             // Store setup executable path if found and DOS-compatible.
             // Some games (e.g. Fallout) ship a Windows SETUP.EXE even though the main
@@ -113,7 +118,9 @@ public class UploadResource {
             game.setBundleFile(bundleFile);
             game.setPlatform(platform);
             // Store the selected executable and the full list for the UI picker
-            String relMain = extractDir.relativize(Path.of(mainExe)).toString().replace('\\', '/');
+            String relMain = mainExe != null
+                ? extractDir.relativize(Path.of(mainExe)).toString().replace('\\', '/')
+                : "";
             game.setExecutable(relMain);
             game.setExecutables(findAllExecutables(extractDir));
             if (setupExe != null) {
@@ -438,11 +445,17 @@ public class UploadResource {
         // Write .jsdos config directly into the extract directory
         Path jsdos = extractDir.resolve(".jsdos");
         Files.createDirectories(jsdos);
-        String relExe = extractDir.relativize(Path.of(exePath)).toString();
         // Find CD images for DOSBox imgmount (mountable via D:, E:, …)
         List<String> cdImages = findCdImages(extractDir);
+        if (exePath != null) {
+            String relExe = extractDir.relativize(Path.of(exePath)).toString();
             Files.writeString(jsdos.resolve("dosbox.conf"), buildDosboxConf(relExe, cdImages));
             Files.write(jsdos.resolve("jsdos.json"), buildJsdosJson(relExe));
+        } else {
+            // CD-only game — no executable, mount CD and show a DOS prompt
+            Files.writeString(jsdos.resolve("dosbox.conf"), buildCdOnlyDosboxConf(cdImages));
+            Files.writeString(jsdos.resolve("jsdos.json"), buildCdOnlyJsdosJson());
+        }
 
         // ZIP it up directly from extractDir (no temp copy)
         try (var zos = new java.util.zip.ZipOutputStream(Files.newOutputStream(bundlePath))) {
@@ -550,6 +563,46 @@ public class UploadResource {
         return json.getBytes(java.nio.charset.StandardCharsets.UTF_8);
     }
 
+    /** Build config for a CD-only game (no executables on the filesystem). */
+    private String buildCdOnlyDosboxConf(List<String> cdImages) {
+        StringBuilder sb = new StringBuilder();
+        sb.append("[sdl]\nautolock=true\nusescancodes=true\noutput=surface\n\n");
+        sb.append("[cpu]\ncore=auto\ncycles=auto\n\n");
+        sb.append("[mixer]\nnosound=false\nrate=44100\n\n");
+        sb.append("[sblaster]\nsbtype=sb16\nirq=7\ndma=1\nhdma=5\n\n");
+        sb.append("[dos]\nxms=true\nems=true\numb=true\n\n");
+        sb.append("[autoexec]\n@echo off\nmount c .\n");
+        // Mount CD images as D:, E:, …
+        Set<String> seenDirs = new java.util.HashSet<>();
+        char driveLetter = 'D';
+        for (String img : cdImages) {
+            int slashIdx = img.lastIndexOf('/');
+            String parentDir = slashIdx >= 0 ? img.substring(0, slashIdx) : "";
+            if (!seenDirs.add(parentDir)) continue;
+            String imgLower = img.toLowerCase();
+            String flags = imgLower.endsWith(".bin") ? " -t cdrom -fs iso" : " -t cdrom";
+            sb.append("imgmount ").append(driveLetter).append(" \"").append(img).append("\"").append(flags).append("\n");
+            driveLetter++;
+        }
+        // Present a DOS prompt — the game runs from the CD
+        sb.append("c:\n");
+        sb.append("echo.\n");
+        sb.append("echo This game runs from the CD-ROM.\n");
+        sb.append("echo Type D: and press Enter, then look for the game executable (e.g. DOTT.EXE).\n");
+        return sb.toString();
+    }
+
+    private byte[] buildCdOnlyJsdosJson() {
+        String json = "{"
+            + "\"autoexec\":{"
+            + "\"options\":{\"script\":{\"value\":\"c:\\\\\"}}"
+            + "},"
+            + "\"output\":{"
+            + "\"options\":{\"autolock\":{\"value\":true}}}"
+            + "}";
+        return json.getBytes(java.nio.charset.StandardCharsets.UTF_8);
+    }
+
     private String buildDosboxConf(String relExe, List<String> cdImages) {
         // DOS uses \\ as path separator; / is a switch character.
         // If the executable is in a subdirectory, cd into it first.