Add platform auto-detection and UI flagging for Windows games

- Game.java: add 'platform' field ('dos'/'windows') + 'isPlayable' helper
- UploadResource.java: detect EXE type by reading PE/NE header during upload
- GameResource.java: allow PATCH to update platform
- GameCard.svelte: show 🪟 Win badge for Windows games (blue style)
- GameDetail.svelte: show platform badge, disable Play for Windows with
  'Unplayable' button; add platform dropdown to edit form
- Play.svelte: show blocking overlay for Windows games with 'Try anyway'
  option for users who want to attempt it

src/main/java/com/dostalgia/Game.java

@@ -23,6 +23,9 @@ public class Game {
 
     private String setupExe;    // relative path to SETUP.EXE (null if none)
 
+    /** Platform type: "dos" for pure DOS, "windows" for Windows 3.x/9x+ (requires emulated Windows), null if unknown. */
+    private String platform;
+
     private Integer igdbId;
     private String igdbCoverId;
 
@@ -85,6 +88,14 @@ public class Game {
     /** Returns true if a SETUP.EXE (or variant) was detected in the game archive. */
     public boolean isHasSetup() { return setupExe != null && !setupExe.isBlank(); }
 
+    public String getPlatform() { return platform; }
+    public void setPlatform(String platform) { this.platform = platform; }
+
+    /** Returns true if the game is a pure DOS executable that can run natively in DOSBox. */
+    public boolean isPlayable() {
+        return ready && (platform == null || "dos".equals(platform));
+    }
+
     public Integer getIgdbId() { return igdbId; }
     public void setIgdbId(Integer igdbId) { this.igdbId = igdbId; }
 

src/main/java/com/dostalgia/GameResource.java

@@ -82,6 +82,7 @@ public class GameResource {
             if (updates.containsKey("publisher")) game.setPublisher((String) updates.get("publisher"));
             if (updates.containsKey("description")) game.setDescription((String) updates.get("description"));
             if (updates.containsKey("rating")) game.setRating(asDouble(updates.get("rating")));
+            if (updates.containsKey("platform")) game.setPlatform((String) updates.get("platform"));
 
             svc.save(game);
             return Response.ok(game).build();

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

@@ -91,9 +91,13 @@ public class UploadResource {
                 createBundle(extractDir, setupExe, setupBundlePath);
             }
 
+            // Detect platform (DOS vs Windows) by reading the main EXE header
+            String platform = detectPlatform(Path.of(mainExe));
+
             // Save metadata
             Game game = new Game(gameId, title);
             game.setBundleFile(bundleFile);
+            game.setPlatform(platform);
             game.setSetupExe(setupExe != null ? extractDir.relativize(Path.of(setupExe)).toString() : null);
             game.setReady(true);
             svc.save(game);
@@ -382,6 +386,64 @@ public class UploadResource {
         return s.replace("\\", "\\\\").replace("\"", "\\\"");
     }
 
+    /**
+     * Detect whether an executable targets DOS or Windows by reading its PE/NE header.
+     * <ul>
+     *   <li>Pure MZ (no NE/PE/LE) → "dos"</li>
+     *   <li>NE (Windows 3.x), PE (Win32), LE (VxD) → "windows"</li>
+     *   <li>Cannot read → null</li>
+     * </ul>
+     */
+    static String detectPlatform(Path exePath) {
+        String name = exePath.getFileName().toString().toLowerCase();
+        // .com and .bat files are always DOS
+        if (name.endsWith(".com") || name.endsWith(".bat")) return "dos";
+
+        try (var fis = Files.newInputStream(exePath)) {
+            byte[] buf = new byte[0x80];
+            int read = fis.readNBytes(buf, 0, buf.length);
+            if (read < 2) return null;
+
+            // Must start with MZ
+            if (buf[0] != 0x4D || buf[1] != 0x5A) return null;
+
+            // Read e_lfanew at offset 0x3C
+            if (read < 0x40) return "dos"; // too short for PE/NE, assume DOS
+            int peOffset = (buf[0x3C] & 0xFF)
+                         | ((buf[0x3D] & 0xFF) << 8)
+                         | ((buf[0x3E] & 0xFF) << 16)
+                         | ((buf[0x3F] & 0xFF) << 24);
+
+            // If signature is beyond our buffer, read it
+            byte sig1, sig2;
+            if (peOffset + 2 <= read) {
+                sig1 = buf[peOffset];
+                sig2 = buf[peOffset + 1];
+            } else {
+                try (var fis2 = Files.newInputStream(exePath)) {
+                    fis2.skip(peOffset);
+                    sig1 = (byte) fis2.read();
+                    sig2 = (byte) fis2.read();
+                }
+            }
+
+            // PE  = Portable Executable (Win32)
+            // NE  = New Executable (Windows 3.x)
+            // LE  = Linear Executable (VxD, OS/2)
+            // LX  = Extended Linear Executable (OS/2 2.x)
+            if ((sig1 == 0x50 && sig2 == 0x45)   // PE
+             || (sig1 == 0x4E && sig2 == 0x45)   // NE
+             || (sig1 == 0x4C && sig2 == 0x45)   // LE
+             || (sig1 == 0x4C && sig2 == 0x58))  // LX
+                return "windows";
+
+            return "dos"; // Plain MZ without known Windows signature
+        } catch (IOException e) {
+            LOG.warning("Could not read EXE header for " + exePath + ": " + e.getMessage());
+            return null;
+        }
+    }
+
     // ─── File Utilities ──────────────────────────────────────────
 
     private void deleteDir(Path dir) throws IOException {