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/fast-uri/.github/workflows/lock-threads.yml

@@ -0,0 +1,19 @@
+name: Lock Threads
+
+on:
+  schedule:
+    - cron: '0 0 1 * *'
+  workflow_dispatch:
+
+concurrency:
+  group: lock
+
+permissions:
+  contents: read
+
+jobs:
+  lock-threads:
+    permissions:
+      issues: write
+      pull-requests: write
+    uses: fastify/workflows/.github/workflows/lock-threads.yml@v6