fix(conversation-assistant): fix Swift compilation errors in APIClient

- Replace force unwrap on non-optional deviceId with if-let pattern
- Remove unused uuidBytes variable declaration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

features/conversation-assistant/macos/Sources/Services/APIClient.swift

@@ -56,14 +56,18 @@ class APIClient {
     }
 
     func verifyDevice(code: String) async throws {
-        guard let deviceId = deviceId else {
+        let currentDeviceId: String
+        if let existingId = deviceId {
+            currentDeviceId = existingId
+        } else {
             // Register first if no device ID
             let (newId, _) = try await registerDevice()
             self.deviceId = newId
+            currentDeviceId = newId
         }
 
         let params: [String: String] = [
-            "deviceId": deviceId!,
+            "deviceId": currentDeviceId,
             "code": code
         ]
 
@@ -118,7 +122,6 @@ class APIClient {
             IOServiceMatching("IOPlatformExpertDevice")
         )
 
-        var uuidBytes: Unmanaged<CFTypeRef>?
         let result = IORegistryEntryCreateCFProperty(
             service,
             "IOPlatformUUID" as CFString,