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/caniuse-lite/dist/lib/supported.js

@@ -0,0 +1,9 @@
+module.exports = {
+  y: 1 << 0,
+  n: 1 << 1,
+  a: 1 << 2,
+  p: 1 << 3,
+  u: 1 << 4,
+  x: 1 << 5,
+  d: 1 << 6
+}