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/shell/AppShell.tsx

@@ -20,7 +20,11 @@ import { toast } from "sonner";
 import {
   closeSession, getMe, listSessions, resumeSession, getSession,
   renameSession, setSessionGroup, archiveSession, unarchiveSession, deleteSession, prewarmRuntime,
+  checkDwhHealth,
 } from "../api/sessions";
+import {
+  Dialog, DialogContent, DialogDescription, DialogFooter, DialogHeader, DialogTitle,
+} from "../components/ui/dialog";
 import type { Principal, SessionScope, SessionSummary } from "../api/types";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { useEffect, useMemo, useRef, useState } from "react";
@@ -89,6 +93,19 @@ export function AppShell() {
   const [stopConfirm, setStopConfirm] = useState(false);
   const [collapsedGroups, setCollapsedGroups] = useState<Record<string, boolean>>({});
   const [renameGroupTarget, setRenameGroupTarget] = useState<string | null>(null);
+  const [dwhDown, setDwhDown] = useState(false);
+  const [dwhChecking, setDwhChecking] = useState(true);
+  const [dwhCheckEpoch, setDwhCheckEpoch] = useState(0);
+  useEffect(() => {
+    let cancelled = false;
+    setDwhChecking(true);
+    checkDwhHealth().then((r) => {
+      if (cancelled) return;
+      setDwhDown(!r.ok);
+      setDwhChecking(false);
+    });
+    return () => { cancelled = true; };
+  }, [dwhCheckEpoch]);
 
   const groups = useMemo(
     () => [...new Set(sessions.map((s) => s.group).filter((g): g is string => !!g))].sort(),
@@ -759,6 +776,26 @@ export function AppShell() {
           }}
         />
       )}
+      {dwhDown && (
+        <Dialog open onOpenChange={() => {}}>
+          <DialogContent showCloseButton={false} className="sm:max-w-md">
+            <DialogHeader>
+              <DialogTitle>Connection unavailable</DialogTitle>
+              <DialogDescription>
+                The database is unreachable. Check the VPN connection and try again.
+              </DialogDescription>
+            </DialogHeader>
+            <DialogFooter>
+              <Button
+                disabled={dwhChecking}
+                onClick={() => setDwhCheckEpoch((e) => e + 1)}
+              >
+                {dwhChecking ? "Checking…" : "Retry"}
+              </Button>
+            </DialogFooter>
+          </DialogContent>
+        </Dialog>
+      )}
     </div>
   );
 }
@@ -770,7 +807,7 @@ function EmptyState() {
         Thoth<span className="text-primary">II</span>
       </span>
       <p className="mt-4 max-w-md text-balance text-[0.95rem] leading-relaxed text-muted-foreground">
-        Build datamarts from your data warehouse through a guided,
+        Build datamarts from your database through a guided,
         human-in-the-loop workflow. Type your question in the box below to begin
         a session.
       </p>