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/.eslintrc

@@ -0,0 +1,28 @@
+{
+	"root": true,
+
+	"extends": "@ljharb",
+
+	"ignorePatterns": ["dist/"],
+
+	"rules": {
+		"eqeqeq": [2, "allow-null"],
+		"id-length": [2, { "min": 1, "max": 30 }],
+		"max-statements": [2, 33],
+		"max-statements-per-line": [2, { "max": 2 }],
+		"no-magic-numbers": [1, { "ignore": [0] }],
+		"no-restricted-syntax": [2, "BreakStatement", "ContinueStatement", "DebuggerStatement", "LabeledStatement", "WithStatement"],
+	},
+
+	"overrides": [
+		{
+			"files": "test/**",
+			"rules": {
+				"no-invalid-this": 1,
+				"max-lines-per-function": 0,
+				"max-statements-per-line": [2, { "max": 3 }],
+				"no-magic-numbers": 0,
+			},
+		},
+	],
+}