feat(chat): add showMessageTimestamps preference atom (localStorage, off by default)

surfsense_web/atoms/chat/show-timestamps.atom.ts

@@ -0,0 +1,11 @@
+import { atomWithStorage } from "jotai/utils";
+
+/**
+ * Per-device preference: show a timestamp under each chat message.
+ *
+ * Off by default to match streaming-AI chat convention (ChatGPT/Claude keep
+ * the message stream clean and put time in the conversation list). Persisted
+ * in localStorage, so it does not sync across devices — acceptable for a
+ * cosmetic display toggle.
+ */
+export const showMessageTimestampsAtom = atomWithStorage<boolean>("chat-show-timestamps:v1", false);