Phase 3: closing-overlay fix + goodbye-composer dedupe

Customer chat refreshSessionStatus now clears sessionExpired carryover so the
goodbye composer renders correctly when re-opening a closing session from
history. Backend /api/shared/chat/:id/info returns goodbye_submitted_by_me;
both apps suppress the composer for the side that has already submitted and
render an awaiting-banner view instead.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/services/closure.service.js

@@ -144,3 +144,12 @@ export const getSessionClosures = async (sessionId) => {
     ORDER BY created_at ASC
   `
 }
+
+export const hasUserSubmittedClosure = async (sessionId, userType) => {
+  const [row] = await sql`
+    SELECT 1 FROM session_closures
+    WHERE session_id = ${sessionId} AND user_type = ${userType}
+    LIMIT 1
+  `
+  return !!row
+}