feat(sdk): wire device_id through messaging and client APIs

Add device_id parameter to fetch, fetch_wait, ack, receive_messages,
and receive_messages_wait SDK functions. QpqClient gains device_id
field with register_device/list_devices/revoke_device convenience
methods. Client REPL passes empty device_id for backwards compat.

crates/quicproquo-sdk/src/client.rs

@@ -23,6 +23,10 @@ pub struct QpqClient {
     session_token: Option<Vec<u8>>,
     /// Local conversation store (SQLCipher).
     conv_store: Option<ConversationStore>,
+    /// Device ID for multi-device support.
+    /// When set, fetch/peek/ack requests include this device_id so the server
+    /// scopes them to the correct per-device queue.
+    device_id: Option<Vec<u8>>,
 }
 
 impl QpqClient {
@@ -37,6 +41,7 @@ impl QpqClient {
             identity_key: None,
             session_token: None,
             conv_store: None,
+            device_id: None,
         }
     }
 
@@ -174,6 +179,45 @@ impl QpqClient {
         self.session_token.as_deref()
     }
 
+    // ── Multi-device ─────────────────────────────────────────────────────────
+
+    /// Set the device ID for this client. Subsequent fetch/peek/ack calls
+    /// will include this ID so the server scopes them to the correct queue.
+    pub fn set_device_id(&mut self, device_id: Vec<u8>) {
+        self.device_id = Some(device_id);
+    }
+
+    /// Get the current device ID, if set.
+    pub fn device_id(&self) -> Option<&[u8]> {
+        self.device_id.as_deref()
+    }
+
+    /// Register this device with the server.
+    /// Sets the local device_id on success.
+    pub async fn register_device(
+        &mut self,
+        device_id: &[u8],
+        device_name: &str,
+    ) -> Result<bool, SdkError> {
+        let rpc = self.rpc.as_ref().ok_or(SdkError::NotConnected)?;
+        let newly_registered =
+            crate::devices::register_device(rpc, device_id, device_name).await?;
+        self.device_id = Some(device_id.to_vec());
+        Ok(newly_registered)
+    }
+
+    /// List all registered devices for this identity.
+    pub async fn list_devices(&self) -> Result<Vec<crate::devices::DeviceInfo>, SdkError> {
+        let rpc = self.rpc.as_ref().ok_or(SdkError::NotConnected)?;
+        crate::devices::list_devices(rpc).await
+    }
+
+    /// Revoke (remove) a registered device.
+    pub async fn revoke_device(&self, device_id: &[u8]) -> Result<bool, SdkError> {
+        let rpc = self.rpc.as_ref().ok_or(SdkError::NotConnected)?;
+        crate::devices::revoke_device(rpc, device_id).await
+    }
+
     // ── Moderation (client-side) ────────────────────────────────────────────
 
     /// Block a user locally. Their messages will be hidden from display.