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/define-properties/.eslintrc

@@ -0,0 +1,19 @@
+{
+	"root": true,
+
+	"extends": "@ljharb",
+
+	"rules": {
+		"id-length": [2, { "min": 1, "max": 35 }],
+		"max-params": [2, 4],
+	},
+
+	"overrides": [
+		{
+			"files": "test/**",
+			"rules": {
+				"max-lines-per-function": 0,
+			},
+		},
+	],
+}