feat: DWH connectivity probe at startup — modal alert within 5s if unreachable

Backend: GET /health/dwh (unauthenticated) calls tht db ping with a 5s
timeout. Frontend: checkDwhHealth() races a 5s timer against the fetch;
on failure a non-dismissable Dialog with Retry appears immediately.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/test/msw.ts

@@ -1,2 +1,5 @@
 import { setupServer } from "msw/node";
-export const server = setupServer();
+import { http, HttpResponse } from "msw";
+export const server = setupServer(
+  http.get("http://localhost:8787/health/dwh", () => HttpResponse.json({ ok: true, detail: "ok" })),
+);