feat: add backend handling for unarchiving all chats

The previous implementation for unarchiving all chats in `ArchivedChatsModal.svelte` was inefficient, as it sent a separate request for each chat, which could potentially overload the server.

This commit introduces a new backend endpoint, `/chats/unarchive/all`, to handle the bulk unarchiving of all chats for a user with a single API call.

The frontend has been updated to use this new endpoint, resolving the performance issue by minimizing the number of requests to the server.

backend/open_webui/routers/chats.py

@@ -361,6 +361,16 @@ async def archive_all_chats(user=Depends(get_verified_user)):
     return Chats.archive_all_chats_by_user_id(user.id)
 
 
+############################
+# UnarchiveAllChats
+############################
+
+
+@router.post("/unarchive/all", response_model=bool)
+async def unarchive_all_chats(user=Depends(get_verified_user)):
+    return Chats.unarchive_all_chats_by_user_id(user.id)
+
+
 ############################
 # GetSharedChatById
 ############################