feat: Add WASM support and browser extension infrastructure

- Add WASM build target and dependencies for all crates.
- Implement IndexedDB-based persistent storage for WASM.
- Create browser extension infrastructure (UI, scripting, etc.).
- Integrate Rhai scripting engine for secure automation.
- Implement user stories and documentation for the extension.

docs/user_stories.md

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+# User Stories: Modular Cryptographic Extension & Scripting
+
+## As a User, I want to...
+
+### Session & Key Management
+- Create a new encrypted keyspace with a password so that only I can access my secrets.
+- Unlock an existing keyspace by entering my password.
+- Create, select, and manage multiple keypairs within a keyspace.
+- Clearly see which keyspace and keypair are currently active in my session.
+
+### Cryptographic Operations
+- Sign and verify messages using my selected keypair.
+- Encrypt and decrypt messages or files using asymmetric cryptography (public/private keypair).
+- Encrypt and decrypt messages or files using symmetric encryption (derived from my keyspace password).
+- Export or back up my keypairs securely.
+
+### EVM Client Actions
+- Connect to an Ethereum provider and check my account balance.
+- Send transactions using my selected keypair.
+
+### Scripting (Rhai)
+- Paste or write a Rhai script into the extension UI and execute it securely.
+- Approve or deny each script execution, with a clear understanding of what the script will access (e.g., signing, sending transactions).
+- See script logs/output in the extension UI.
+
+### Security & Permissions
+- Be prompted for approval before any script can access my keypair or perform sensitive operations.
+- See a clear audit trail/log of all cryptographic and scripting actions performed in my session.
+
+### WebSocket Integration (Future)
+- Connect to a server using my keypair's public key and receive Rhai scripts from the server.
+- Review and approve/reject incoming scripts, with clear metadata (title, description, tags).
+- For remote scripts, sign the script hash and send my signature to the server as consent.
+- For local scripts, execute them in the extension and have the results reported back to the server.
+
+### UI/UX
+- Use a fast, modern, and intuitive extension interface, with dark mode support.
+- Always know the current security state (locked/unlocked, connected/disconnected, etc.).
+
+---
+
+## As a Developer, I want to...
+
+- Expose all vault and EVM client APIs to WASM so they are callable from JavaScript/TypeScript.
+- Provide ergonomic Rust-to-Rhai bindings for all key cryptographic and EVM actions.
+- Ensure clear error reporting and logging for all extension and scripting operations.
+- Write tests for both WASM and native environments.
+- Easily add new cryptographic algorithms, providers, or scripting APIs as the system evolves.