[FEATURE]: feat: add TypeScript type definitions (.d.ts) for config options

### Feature and its Use Cases

What change I'm going to make:
The repo is primarily TypeScript with React/Next.js wrappers, but currently doesn't ship proper .d.ts type definitions for the package's config options. I'd like to add type definitions for:

buttonStyle
theme
platforms
onShare (callback)
onCopy (callback)

Why this matters:
Without proper types, TS consumers lose autocomplete and type-checking on these props, which goes against the package being TS-first.

Plan:
I'll create the .d.ts files, ensure they're exported correctly, and test them against a sample React/Next.js consumer to confirm resolution works.

### Additional Context

no

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates