Add cdk-wasm crate with browser wallet support

[crodas]

Introduce the cdk-wasm crate providing WebAssembly bindings for the CDK wallet. This enables running Cashu wallets directly in browsers and Node.js environments.

Key components:

• Wallet and WalletRepository wasm_bindgen bindings
• LocalStorage-backed database with swappable storage backend
• Token parsing, encoding, and inspection
• Full WASM type system mirroring cdk-common types
• HTTP client implementation for wasm32 (fetch API)
• Demo pages (www/) for browser testing
• CI workflow for wasm32 build checks

Also adapts cdk-http-client to compile for wasm32 by gating native dependencies and adding a fetch-based transport behind the wasm32 target.

Description

---

Notes to the reviewers

---

Suggested CHANGELOG Updates

CHANGED

ADDED

REMOVED

FIXED

---

Checklist

• I followed the code style guidelines
• I ran just final-check before committing

[thesimplekid]

We need to split of this PR into multiple smaller PRs so it can be reviewed. It seems to do multiple things; change deps in signatory, modify the http client as well as introduce js bindings. The changes to the http client should be its own pr as they do not depend on the bindings.

On the bindings I have a strong preference for not redefining all these wasm types that we already have defined in cdk-ffi, it creates much more code to maintain. I know other projects have had recent success reusing their uniffi bindings with wasm using this project. I think this would be better for us since we already support uniffi. Did you look into wasm support via uniffi and there is a specefic reason we cannot use it?

[crodas]

Closed in flavour of #1665