The mainnet RPC endpoint hardcodes a third-party provider

[Abioladory123]

Summary

NETWORKS.mainnet points at a specific commercial RPC provider's URL. Anyone
calling getNetwork("mainnet") silently routes their traffic through that
provider, with whatever rate limits and availability it happens to have, and
without having chosen it.

networkFromEnv already exists as the override, so the fix is mostly about not
making the surprising choice the default.

Acceptance criteria

• The mainnet default either requires an explicit RPC URL or documents the
  provider prominently at the point of use.
• The README states that mainnet traffic should use an endpoint the consumer
  controls.
• Testnet and local behaviour are unchanged.

Getting started

Fork this repository, clone your fork, and add this repo as upstream:

git clone https://github.com/<your-username>/novatip-sdk.git
cd novatip-sdk
git remote add upstream https://github.com/Novatip/novatip-sdk.git
npm install

Create a branch for this issue:

git checkout -b fix/mainnet-rpc-default

Suggested commit message:

fix: reconsider the hardcoded mainnet RPC

Run npm run lint, npm run typecheck, and npm run build before pushing, then open a pull request from your fork against main and link this issue. See the contributing guide for the full workflow.