chore: synchronize pending app/backend updates and repository hygiene

Includes current frontend and backend work in progress and removes generated artifacts from tracking to keep the repository clean for дальнейшая разработка.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/ping-probe.ts

@@ -0,0 +1,5 @@
+/**
+ * Порог RTT (мс) для визуала «деградации» на `/uptime` — должен совпадать с
+ * `PING_PROBE_WARN_RTT_MS` в `backend/src/constants/ping-probe.ts` (логика `parsePing`).
+ */
+export const PING_PROBE_WARN_RTT_MS = 120