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.

vault/src/session_singleton.rs

@@ -0,0 +1,12 @@
+//! WASM session singleton for the vault crate
+//! This file defines the global SessionManager singleton for WASM builds.
+
+use once_cell::unsync::Lazy;
+use std::cell::RefCell;
+use crate::session::SessionManager;
+use kvstore::wasm::WasmStore;
+
+// Thread-local singleton for WASM session management
+thread_local! {
+    pub static SESSION_MANAGER: Lazy<RefCell<Option<SessionManager<WasmStore>>>> = Lazy::new(|| RefCell::new(None));
+}