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-keys/.eslintrc

@@ -0,0 +1,17 @@
+{
+	"root": true,
+
+	"extends": "@ljharb",
+
+	"rules": {
+		"complexity": [2, 23],
+		"id-length": [2, { "min": 1, "max": 40 }],
+		"max-params": [2, 3],
+		"max-statements": [2, 23],
+		"max-statements-per-line": [2, { "max": 2 }],
+		"no-extra-parens": [1],
+		"no-invalid-this": [1],
+		"no-restricted-syntax": [2, "BreakStatement", "ContinueStatement", "LabeledStatement", "WithStatement"],
+		"operator-linebreak": [2, "after"]
+	}
+}