feat: add TranscriptionLogger for append-only session log

MyVoxtral/MyVoxtral/Utilities/TranscriptionLogger.swift

@@ -0,0 +1,27 @@
+import Foundation
+
+struct TranscriptionLogger {
+    private static var logFileURL: URL {
+        let appSupport = FileManager.default.urls(for: .applicationSupportDirectory, in: .userDomainMask).first!
+        let dir = appSupport.appendingPathComponent("MyVoxtral", isDirectory: true)
+        try? FileManager.default.createDirectory(at: dir, withIntermediateDirectories: true)
+        return dir.appendingPathComponent("transcription.log")
+    }
+
+    static func append(text: String) {
+        let timestamp = ISO8601DateFormatter().string(from: Date())
+        let entry = "[\(timestamp)]\n\(text)\n---\n\n"
+
+        guard let data = entry.data(using: .utf8) else { return }
+
+        if FileManager.default.fileExists(atPath: logFileURL.path) {
+            if let handle = try? FileHandle(forWritingTo: logFileURL) {
+                handle.seekToEndOfFile()
+                handle.write(data)
+                handle.closeFile()
+            }
+        } else {
+            try? data.write(to: logFileURL)
+        }
+    }
+}