fix: preserve chat input text across window resize (#4717)

Adds a Recoil atom (chatInputDraftAtom) to store the chat input draft,
which persists the text when the chat component unmounts/remounts during
mobile/desktop mode transitions.

Changes:
- Add chatInputDraftAtom to ClientConfigStore
- Update ChatTextField to save draft on input change
- Restore draft when component mounts
- Clear draft when message is sent

Closes #3615

web/components/stores/ClientConfigStore.tsx

@@ -73,6 +73,12 @@ export const chatAuthenticatedAtom = atom<boolean>({
   default: false,
 });
 
+// Stores chat input draft to preserve text across mobile/desktop mode switches
+export const chatInputDraftAtom = atom<string>({
+  key: 'chatInputDraftAtom',
+  default: '',
+});
+
 export const websocketServiceAtom = atom<WebsocketService>({
   key: 'websocketServiceAtom',
   default: null,