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/fast-json-stable-stringify/.eslintrc.yml

@@ -0,0 +1,26 @@
+extends: eslint:recommended
+env:
+  node: true
+  browser: true
+rules:
+  block-scoped-var: 2
+  callback-return: 2
+  dot-notation: 2
+  indent: 2
+  linebreak-style: [2, unix]
+  new-cap: 2
+  no-console: [2, allow: [warn, error]]
+  no-else-return: 2
+  no-eq-null: 2
+  no-fallthrough: 2
+  no-invalid-this: 2
+  no-return-assign: 2
+  no-shadow: 1
+  no-trailing-spaces: 2
+  no-use-before-define: [2, nofunc]
+  quotes: [2, single, avoid-escape]
+  semi: [2, always]
+  strict: [2, global]
+  valid-jsdoc: [2, requireReturn: false]
+  no-control-regex: 0
+  no-useless-escape: 2