refactor: centralize main session key normalization in apps

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
+    }
+}