fix: 3 bugs in Electron export pipeline

Bug 1: RENDERS_DIR mismatch (root cause)
- server.ts had hardcoded renders dir, now reads BRADLY_RENDERS_DIR env
- renderQueue.ts saves to ~/Library/.../Bradly/renders/
- server.ts now serves from the same directory

Bug 2: Upload origin pointing to wrong port
- uploadBlobContent.ts used window.location.origin (Vite 5173)
- Remotion bundler needs Express origin (3000) to access media
- Added getExpressOrigin() helper that detects Electron

Bug 3: Batch ZIP export using file-saver (doesn't work in Electron)
- Added saveBlobFile IPC method (preload + main)
- batchExporter.ts now uses native save dialog in Electron
- Web mode falls back to file-saver

src/utils/batchExporter.ts

@@ -198,7 +198,19 @@ export async function exportBatchAsZip(
     .replace(/\s+/g, '_')
     .replace(/[^a-zA-Z0-9._-]/g, '');
 
-  saveAs(zipBlob, zipName);
+  // In Electron, use native save dialog
+  const electronAPI = (typeof window !== 'undefined') ? (window as any).electronAPI : null;
+  if (electronAPI?.saveBlobFile) {
+    const arrayBuffer = await zipBlob.arrayBuffer();
+    await electronAPI.saveBlobFile(
+      new Uint8Array(arrayBuffer),
+      zipName,
+      [{ name: 'ZIP Archive', extensions: ['zip'] }],
+    );
+  } else {
+    // Web fallback
+    saveAs(zipBlob, zipName);
+  }
 
   onProgress?.({ current: total, total, status: 'done' });
 }