stability: Sprint 01 — production stability (dev runner, test gate, Docker UI, cpu_percent)

- Fix dev deploy runner label (ubuntu-latest → nas)
- Add backend + frontend test gate to dev deploy (tests must pass before deploy)
- Add error handling UI to Docker.svelte (error state with retry button)
- Fix psutil.cpu_percent always returning 0 on first call (interval=0 → 0.1)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

dashboard/backend/routers/system.py

@@ -50,7 +50,7 @@ def system_stats():
             except Exception:
                 pass
     mem = psutil.virtual_memory()
-    cpu_pct = psutil.cpu_percent(interval=0)  # Non-blocking, uses cached value
+    cpu_pct = psutil.cpu_percent(interval=0.1)
     load_1, load_5, load_15 = psutil.getloadavg()
     uptime_s = time.time() - psutil.boot_time()