Make the bot POTD statement more fun by randomizing the daily message

The discord bot that publishes the daily Leetcode problem publishes the link and an additional message per difficulty in https://github.com/code-chronicles-code/leetcode-curriculum/blob/main/workspaces/post-leetcode-potd-to-discord/src/getPotdMessage.ts.

We can make the message more fun by having more random messages for each difficulty level.

Start by reading workspaces/post-leetcode-potd-to-discord/src/getPotdMessage.ts, especially how the additional message is selected for each difficulty level. Add randomized alternatives while preserving the existing daily problem link and difficulty-specific behavior. Done means the bot can produce more than one suitable message for each difficulty level.

Written by the indexing model from the issue text.