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>

backend/src/tht/tht-runner.ts

@@ -154,7 +154,7 @@ export class ThtRunner {
    * cleanly. Timed out to keep POST /sessions responsive when the host hangs.
    */
   async dbPing(workspace?: string): Promise<{ ok: boolean; detail: string }> {
-    const { code, stdout, stderr } = await this.run(["db", "ping"], workspace, 10_000);
+    const { code, stdout, stderr } = await this.run(["db", "ping"], workspace, 5_000);
     if (code === 0) return { ok: true, detail: stdout.trim() };
     return { ok: false, detail: (stderr || stdout).trim() };
   }