truncateMessage's default length is unrelated to the contract's limit

[Abioladory123]

Summary

truncateMessage defaults to 80 characters. The contract caps a tip message at
280 bytes, and the frontend enforces its own separate number. Three layers, three
unrelated values, none deriving from the others.

80 may be a perfectly reasonable display default, but as an unexplained literal
it reads like a limit rather than a display choice, and there is nothing
connecting it to what the contract actually permits.

Acceptance criteria

• The default is either derived from an exported message-length constant or
  documented as a display-only choice with its reasoning.
• The relationship to the contract's byte limit is stated in the doc
  comment, including that one counts bytes and the other characters.
• Tests cover a message at the boundary and one past it.

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/truncate-message-default

Suggested commit message:

fix: relate the truncation default to the message limit

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.