fix: pass format as first argument to image.write()

The correct ImageMagick WASM API is:
image.write(format, callback)

Where the format is passed as the first argument and the callback
receives the encoded data. This should produce valid WebP with
correct RIFF headers.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/converters/imagemagickService.ts

@@ -39,9 +39,6 @@ export async function convertWithImageMagick(
     let result: Uint8Array | undefined;
 
     await ImageMagick.read(inputData, (image) => {
-      // Set output format
-      image.format = outputFormatEnum;
-
       // Apply quality setting if specified
       if (options.imageQuality !== undefined) {
         image.quality = options.imageQuality;
@@ -68,8 +65,8 @@ export async function convertWithImageMagick(
 
       if (onProgress) onProgress(70);
 
-      // Write the image data in the specified format
-      image.write((data) => {
+      // Write the image data with format
+      image.write(outputFormatEnum, (data) => {
         result = data;
       });