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/workbox-cacheable-response/build/workbox-cacheable-response.prod.js

@@ -0,0 +1,2 @@
+this.workbox=this.workbox||{},this.workbox.cacheableResponse=function(s){"use strict";try{self["workbox:cacheable-response:7.4.0"]&&_()}catch(s){}class t{constructor(s={}){this.ss=s.statuses,this.ts=s.headers}isResponseCacheable(s){let t=!0;return this.ss&&(t=this.ss.includes(s.status)),this.ts&&t&&(t=Object.keys(this.ts).some((t=>s.headers.get(t)===this.ts[t]))),t}}return s.CacheableResponse=t,s.CacheableResponsePlugin=class{constructor(s){this.cacheWillUpdate=async({response:s})=>this.hs.isResponseCacheable(s)?s:null,this.hs=new t(s)}},s}({});
+//# sourceMappingURL=workbox-cacheable-response.prod.js.map