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

kvstore/src/traits.rs

@@ -0,0 +1,26 @@
+//! Async trait for key-value storage
+
+use crate::error::Result;
+
+#[cfg_attr(target_arch = "wasm32", async_trait::async_trait(?Send))]
+#[cfg_attr(not(target_arch = "wasm32"), async_trait::async_trait)]
+/// Async key-value store interface.
+///
+/// For native (non-wasm32) backends, implementers should be `Send + Sync` to support runtime-agnostic async usage.
+/// For WASM (wasm32) backends, `Send + Sync` is not required and types may not implement them.
+///
+/// Methods:
+/// - get
+/// - set
+/// - remove (was delete)
+/// - contains_key (was exists)
+/// - keys
+/// - clear
+pub trait KVStore {
+    async fn get(&self, key: &str) -> Result<Option<Vec<u8>>>;
+    async fn set(&self, key: &str, value: &[u8]) -> Result<()>;
+    async fn remove(&self, key: &str) -> Result<()>;
+    async fn contains_key(&self, key: &str) -> Result<bool>;
+    async fn keys(&self) -> Result<Vec<String>>;
+    async fn clear(&self) -> Result<()>;
+}