chore(audiobooks): reuse existing now var in login envelope

Replaces second time.Now() call with now.UnixMilli() using the now var
already created higher up in completeLogin for token TTL math.

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

internal/audiobooks/abs/login.go

@@ -197,7 +197,7 @@ func (h *Handler) completeLogin(w http.ResponseWriter, r *http.Request, userID,
 		libraryMaps = append(libraryMaps, audiobookLibraryMap(lib))
 	}
 
-	nowMs := time.Now().UnixMilli()
+	nowMs := now.UnixMilli()
 
 	// Real ABS clients pattern-match on a richer login envelope than the
 	// minimum we previously sent. The added fields (itemTagsAccessible,