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/internal-slot/index.d.ts

@@ -0,0 +1,12 @@
+declare namespace SLOT {
+	type InternalSlot = string; // `[[${string}]]`; // TODO: restrict this to require the brackets
+}
+
+declare const SLOT: {
+	assert(O: object, slot: SLOT.InternalSlot): void;
+	get(O: object, slot: SLOT.InternalSlot): unknown;
+	set(O: object, slot: SLOT.InternalSlot, value?: unknown): void;
+	has(O: object, slot: SLOT.InternalSlot): boolean;
+}
+
+export = SLOT;