fix: properly escape JSON string values (newlines, control chars)

escapeJsonValue only escaped backslash and double-quote, but the
autoexec script contains literal newlines which are invalid in JSON
strings. Now escapes \n, \r, \t and all control characters < 0x20.

Fixes JSON.parse error in Theme Hospital: 'bad character in string literal'

src/main/java/com/dostalgia/GameService.java

@@ -299,7 +299,24 @@ public class GameService {
     }
 
     private static String escapeJsonValue(String s) {
-        return s.replace("\\", "\\\\").replace("\"", "\\\"");
+        StringBuilder sb = new StringBuilder(s.length());
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            switch (c) {
+                case '\\': sb.append("\\\\"); break;
+                case '"': sb.append("\\\""); break;
+                case '\n': sb.append("\\n"); break;
+                case '\r': sb.append("\\r"); break;
+                case '\t': sb.append("\\t"); break;
+                default:
+                    if (c < 0x20) {
+                        sb.append(String.format("\\u%04x", (int) c));
+                    } else {
+                        sb.append(c);
+                    }
+            }
+        }
+        return sb.toString();
     }
 
     /** Quick platform detection for a given executable inside a .jsdos bundle. */

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

@@ -623,7 +623,7 @@ public class UploadResource {
         script.append(parts[1]);
         String json = "{"
             + "\"autoexec\":{"
-            + "\"options\":{\"script\":{\"value\":\"" + escapeJson(script.toString()) + "\"}}"
+            + "\"options\":{\"script\":{\"value\":\"" + escapeJsonValue(script.toString()) + "\"}}"
             + "},"
             + "\"output\":{"
             + "\"options\":{\"autolock\":{\"value\":true}}}"
@@ -762,8 +762,25 @@ public class UploadResource {
         return sb.toString();
     }
 
-    private String escapeJson(String s) {
-        return s.replace("\\", "\\\\").replace("\"", "\\\"");
+    private static String escapeJsonValue(String s) {
+        StringBuilder sb = new StringBuilder(s.length());
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+            switch (c) {
+                case '\\': sb.append("\\\\"); break;
+                case '"': sb.append("\\\""); break;
+                case '\n': sb.append("\\n"); break;
+                case '\r': sb.append("\\r"); break;
+                case '\t': sb.append("\\t"); break;
+                default:
+                    if (c < 0x20) {
+                        sb.append(String.format("\\u%04x", (int) c));
+                    } else {
+                        sb.append(c);
+                    }
+            }
+        }
+        return sb.toString();
     }
 
     /**