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>

backend/src/routes/workforce.js

@@ -58,6 +58,7 @@ export async function workforceRoutes(app) {
       await upsertWorkforceShifts(dailyRows)
       return { ok: true, from, to, dates_synced: dailyRows.length }
     } catch (err) {
+      req.log.error({ err: err.message }, 'workforce sync failed')
       const status = err.message.includes('not configured') ? 503 : 502
       return reply.status(status).send({ error: err.message })
     }