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/@trickfilm400/rollup-plugin-off-main-thread/.travis.yml

@@ -0,0 +1,8 @@
+sudo: required
+
+services:
+  - docker
+
+script:
+  - docker build -t omt .
+  - docker run --rm -v `pwd`:/usr/src omt