feat(pos-pwa): pOS: make TPV installable PWA and launch fullscreen

work/artifacts/POS-PWA/qa.json

@@ -0,0 +1,12 @@
+{
+  "feature_id": "POS-PWA",
+  "agent": "qa",
+  "stage": "qa_gate",
+  "verdict": "APPROVED",
+  "qa_check": "qa",
+  "summary": "Build clean. manifest.webmanifest visible in build output.",
+  "test_results": {
+    "automated": ["cd project/apps/pos && npm run build ✅", "git diff --check ✅"]
+  },
+  "timestamp": "2026-08-26T18:36:30Z"
+}