Add sync error logging, extend error flash, fix unload saves

- Log workforce sync errors to backend stdout so they appear in docker logs
- Extend error flash from 5s to 15s so errors are readable
- Replace sendBeacon (POST-only) with keepalive fetch (PUT) on unload —
  sendBeacon was hitting 404s on all three unload saves

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/node_modules/cross-spawn/lib/util/readShebang.js

@@ -0,0 +1,23 @@
+'use strict';
+
+const fs = require('fs');
+const shebangCommand = require('shebang-command');
+
+function readShebang(command) {
+    // Read the first 150 bytes from the file
+    const size = 150;
+    const buffer = Buffer.alloc(size);
+
+    let fd;
+
+    try {
+        fd = fs.openSync(command, 'r');
+        fs.readSync(fd, buffer, 0, size, 0);
+        fs.closeSync(fd);
+    } catch (e) { /* Empty */ }
+
+    // Attempt to extract shebang (null is returned if not a shebang)
+    return shebangCommand(buffer.toString());
+}
+
+module.exports = readShebang;