[TASK] Replace Big Number usages from @polkadot/util across dapps

[vutuanlinh2k2]

Description

• All the dapps' code are heavily rely on BN provided by @polkadot/util, which need to be migrated to bn.js

[drewstone]

Done in v2, take on something more closer to live testing please (test and fix the leaderboard use the setup-local-env.sh script) ask the agent how to run things. Be proactive if you're confused to ask the agent how to run the setup.