refactor: centralize main session key normalization in apps

apps/ios/Sources/Model/NodeAppModel.swift

@@ -330,8 +330,7 @@ final class NodeAppModel {
             let ui = config["ui"] as? [String: Any]
             let raw = (ui?["seamColor"] as? String)?.trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
             let session = config["session"] as? [String: Any]
-            let rawMainKey = (session?["mainKey"] as? String)?.trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
-            let mainKey = rawMainKey.isEmpty ? "main" : rawMainKey
+            let mainKey = SessionKey.normalizeMainKey(session?["mainKey"] as? String)
             await MainActor.run {
                 self.seamColorHex = raw.isEmpty ? nil : raw
                 self.mainSessionKey = mainKey

apps/ios/Sources/SessionKey.swift

@@ -0,0 +1,8 @@
+import Foundation
+
+enum SessionKey {
+    static func normalizeMainKey(_ raw: String?) -> String {
+        let trimmed = (raw ?? "").trimmingCharacters(in: .whitespacesAndNewlines)
+        return trimmed.isEmpty ? "main" : trimmed
+    }
+}

apps/ios/Sources/Voice/TalkModeManager.swift

@@ -651,8 +651,7 @@ final class TalkModeManager: NSObject {
             guard let config = json["config"] as? [String: Any] else { return }
             let talk = config["talk"] as? [String: Any]
             let session = config["session"] as? [String: Any]
-            let rawMainKey = (session?["mainKey"] as? String)?.trimmingCharacters(in: .whitespacesAndNewlines) ?? ""
-            self.mainSessionKey = rawMainKey.isEmpty ? "main" : rawMainKey
+            self.mainSessionKey = SessionKey.normalizeMainKey(session?["mainKey"] as? String)
             self.defaultVoiceId = (talk?["voiceId"] as? String)?.trimmingCharacters(in: .whitespacesAndNewlines)
             if let aliases = talk?["voiceAliases"] as? [String: Any] {
                 var resolved: [String: String] = [:]