feat: Add basic project structure and initial crates

- Added basic project structure with workspace and crates:
  `kvstore`, `vault`, `evm_client`, `cli_app`, `web_app`.
- Created initial `Cargo.toml` files for each crate.
- Added placeholder implementations for key components.
- Included initial documentation files (`README.md`, architecture
  docs, repo structure).
- Included initial implementaion for kvstore crate(async API, backend abstraction, separation of concerns, WASM/native support, testability)
- Included native and browser tests for the kvstore crate

evm_client/src/lib.rs

@@ -0,0 +1,23 @@
+//! evm_client: EVM RPC client and integration
+
+
+
+
+#[derive(Debug, thiserror::Error)]
+pub enum EvmError {
+    #[error("RPC error: {0}")]
+    Rpc(String),
+    #[error("Vault error: {0}")]
+    Vault(String),
+}
+
+pub struct EvmClient {
+    // ... fields for RPC, vault, etc.
+}
+
+impl EvmClient {
+    pub async fn connect(_rpc_url: &str) -> Result<Self, EvmError> {
+        todo!("Implement connect")
+    }
+    // ... other API stubs
+}