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

server.ts

@@ -190,7 +190,7 @@ export async function createExpressApp() {
   });
 
   // ═══ Render Queue ═══
-  const RENDERS_DIR = path.join(process.cwd(), "renders");
+  const RENDERS_DIR = process.env.BRADLY_RENDERS_DIR || path.join(process.cwd(), "renders");
   if (!fs.existsSync(RENDERS_DIR)) {
     fs.mkdirSync(RENDERS_DIR, { recursive: true });
   }