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

vault/src/lib.rs

@@ -0,0 +1,27 @@
+//! vault: Cryptographic keyspace and operations
+
+
+use kvstore::KVStore;
+
+#[derive(Debug, thiserror::Error)]
+pub enum VaultError {
+    #[error("Storage error: {0}")]
+    Storage(String),
+    #[error("Crypto error: {0}")]
+    Crypto(String),
+    #[error("Unauthorized")]
+    Unauthorized,
+}
+
+pub struct Vault<S: KVStore + Send + Sync> {
+    storage: S,
+    // ... other fields
+}
+
+impl<S: KVStore + Send + Sync> Vault<S> {
+    /// Creates a new keyspace. Implementation pending.
+    pub async fn create_keyspace(_dummy: ()) -> Result<(), VaultError> {
+        todo!("Implement create_keyspace")
+    }
+    // ... other API stubs
+}