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/object.assign/.editorconfig

@@ -0,0 +1,20 @@
+root = true
+
+[*]
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+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+max_line_length = 150
+
+[CHANGELOG.md]
+indent_style = space
+indent_size = 2
+
+[*.json]
+max_line_length = off
+
+[Makefile]
+max_line_length = off