feat(wasm): secure device API (private keys never cross to JS)

- register_device() generates signing + deploy keypairs via core device
  module, stores them in DEVICE_STATE (once_cell Lazy<Mutex>), and
  returns only public keys to JS
- sign_for_git() signs data using the internal signing key
- get_device_info() returns name and public keys; returns null if not
  registered
- clear_device() zeroes and drops device state (logout / re-registration)
- Removed generate_device_keypair() which exposed raw private key bytes

Fixes audit I5: private key material no longer crosses the WASM boundary.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/relicario-wasm/src/device.rs

@@ -0,0 +1,71 @@
+//! WASM device key management -- private keys never cross to JS.
+
+use std::sync::Mutex;
+
+use once_cell::sync::Lazy;
+use zeroize::Zeroizing;
+
+use relicario_core::device as core_device;
+
+/// In-memory device key storage (private keys held in WASM linear memory).
+static DEVICE_STATE: Lazy<Mutex<Option<DeviceState>>> = Lazy::new(|| Mutex::new(None));
+
+struct DeviceState {
+    name: String,
+    signing_private: Zeroizing<String>,
+    signing_public: String,
+    /// Deploy key stored for future SSH git operations; not yet used for signing.
+    #[allow(dead_code)]
+    deploy_private: Zeroizing<String>,
+    deploy_public: String,
+}
+
+/// Register a new device, storing the keypairs internally and returning
+/// only the public keys. Private keys never leave WASM memory.
+pub fn register_device(name: &str) -> Result<(String, String), String> {
+    let (signing_priv, signing_pub) =
+        core_device::generate_keypair().map_err(|e| e.to_string())?;
+    let (deploy_priv, deploy_pub) =
+        core_device::generate_keypair().map_err(|e| e.to_string())?;
+
+    let state = DeviceState {
+        name: name.to_string(),
+        signing_private: signing_priv,
+        signing_public: signing_pub.clone(),
+        deploy_private: deploy_priv,
+        deploy_public: deploy_pub.clone(),
+    };
+
+    *DEVICE_STATE.lock().unwrap() = Some(state);
+
+    Ok((signing_pub, deploy_pub))
+}
+
+/// Sign `data` using the registered device's signing key.
+/// Returns a base64-encoded signature.
+pub fn sign_for_git(data: &[u8]) -> Result<String, String> {
+    let guard = DEVICE_STATE.lock().unwrap();
+    let state = guard
+        .as_ref()
+        .ok_or_else(|| "no device registered".to_string())?;
+
+    core_device::sign(&state.signing_private, data).map_err(|e| e.to_string())
+}
+
+/// Return current device info: (name, signing_public_key, deploy_public_key).
+/// Returns None if no device has been registered in this session.
+pub fn get_device_info() -> Option<(String, String, String)> {
+    let guard = DEVICE_STATE.lock().unwrap();
+    guard.as_ref().map(|s| {
+        (
+            s.name.clone(),
+            s.signing_public.clone(),
+            s.deploy_public.clone(),
+        )
+    })
+}
+
+/// Clear device state (call on logout or before re-registration).
+pub fn clear_device() {
+    *DEVICE_STATE.lock().unwrap() = None;
+}