Add a Lobstr wallet adapter

[Abioladory123]

Summary

WalletAdapter defines a wallet-agnostic interface, and Freighter and xBull
implement it. Freighter is a desktop browser extension, which means the tip flow
does not work on the phone most supporters actually carry.

Lobstr is widely used on mobile, and the interface is already shaped for this:
two worked implementations exist to follow.

Acceptance criteria

• A Lobstr adapter implements the full WalletAdapter interface, including
  availability detection.
• It follows the optional-dependency pattern the existing adapters use, so
  consumers without it installed are unaffected.
• It is exported from the package entry point and documented alongside the
  other adapters.

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 feat/lobstr-adapter

Suggested commit message:

feat: add a Lobstr wallet adapter

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.