fix: jsonata $base64decode/$base64encode in Web Worker

The jsonata library checks `typeof window !== 'undefined'` to decide
whether to use browser `atob`/`btoa` or Node.js `Buffer.from` for
base64 operations. Since CyberChef runs operations in a Web Worker
where `window` is undefined, it falls back to `global.Buffer` which
also does not exist, causing "Cannot read properties of undefined
(reading 'from')".

Fix by registering custom base64 functions on the expression that use
`atob`/`btoa` directly, which are available in both browser and Web
Worker scopes.

Closes #2063

src/core/operations/Jsonata.mjs

@@ -51,6 +51,18 @@ class JsonataQuery extends Operation {
 
         try {
             const expression = jsonata(query);
+            // Override built-in base64 functions which fail in Web Worker
+            // context where `window` is undefined. The jsonata library falls
+            // back to `global.Buffer` which also does not exist in workers.
+            // `atob`/`btoa` are available in both browser and worker scopes.
+            expression.registerFunction("base64decode", (str) => {
+                if (typeof str === "undefined") return undefined;
+                return atob(str);
+            }, "<s-:s>");
+            expression.registerFunction("base64encode", (str) => {
+                if (typeof str === "undefined") return undefined;
+                return btoa(str);
+            }, "<s-:s>");
             result = await expression.evaluate(jsonObj);
         } catch (err) {
             throw new OperationError(

tests/operations/tests/Jsonata.mjs

@@ -548,4 +548,27 @@ TestRegister.addTests([
             },
         ],
     },
+    // Base64 functions (issue #2063)
+    {
+        name: "Jsonata: $base64decode",
+        input: "{}",
+        expectedOutput: '"Hello World!"',
+        recipeConfig: [
+            {
+                op: "Jsonata Query",
+                args: ['$base64decode("SGVsbG8gV29ybGQh")'],
+            },
+        ],
+    },
+    {
+        name: "Jsonata: $base64encode",
+        input: "{}",
+        expectedOutput: '"SGVsbG8gV29ybGQh"',
+        recipeConfig: [
+            {
+                op: "Jsonata Query",
+                args: ['$base64encode("Hello World!")'],
+            },
+        ],
+    },
 ]);