Add daily file logger target selection

Adds backend/client daily log writing with separate log prefixes and target-aware log directories.

Services/Logging/FileLogTarget.cs

@@ -0,0 +1,7 @@
+namespace UtopiaCanteenSystem.Services.Logging;
+
+public enum FileLogTarget
+{
+    Backend,
+    Client
+}

Services/Logging/FileLogger.cs

@@ -0,0 +1,99 @@
+using System.IO;
+using System.Text;
+
+namespace UtopiaCanteenSystem.Services.Logging;
+
+/// <summary>
+/// Thread-safe daily file logger. Never throws to callers.
+/// </summary>
+public static class FileLogger
+{
+    private static readonly object Lock = new();
+    private static FileLogTarget? _target;
+
+    public static bool IsConfigured => _target.HasValue;
+
+    public static void ConfigureBackend() => Configure(FileLogTarget.Backend);
+
+    public static void ConfigureClient() => Configure(FileLogTarget.Client);
+
+    public static void Configure(FileLogTarget target)
+    {
+        _target = target;
+        var dir = GetLogsDirectory();
+        try
+        {
+            Directory.CreateDirectory(dir);
+        }
+        catch
+        {
+            // Ignore
+        }
+
+        Info("FileLogger", $"Logging initialized. Directory={dir}");
+    }
+
+    public static string GetLogsDirectory() =>
+        _target == FileLogTarget.Client
+            ? LogPaths.ClientLogsDirectory
+            : LogPaths.BackendLogsDirectory;
+
+    public static void Debug(string component, string message) => Write("DEBUG", component, message, null);
+
+    public static void Info(string component, string message) => Write("INFO", component, message, null);
+
+    public static void Warn(string component, string message, Exception? ex = null) =>
+        Write("WARN", component, message, ex);
+
+    public static void Error(string component, string message, Exception? ex = null) =>
+        Write("ERROR", component, message, ex);
+
+    private static void Write(string level, string component, string message, Exception? ex)
+    {
+        if (!_target.HasValue)
+            return;
+
+        try
+        {
+            var dir = GetLogsDirectory();
+            var prefix = _target == FileLogTarget.Client ? "client" : "backend";
+            var file = Path.Combine(dir, $"{prefix}-{DateTime.Now:yyyy-MM-dd}.log");
+            var line = FormatLine(level, component, message, ex);
+
+            lock (Lock)
+            {
+                Directory.CreateDirectory(dir);
+                File.AppendAllText(file, line, Encoding.UTF8);
+            }
+        }
+        catch
+        {
+            // Never crash the app because logging failed.
+        }
+    }
+
+    private static string FormatLine(string level, string component, string message, Exception? ex)
+    {
+        var sb = new StringBuilder();
+        sb.Append($"{DateTime.Now:yyyy-MM-dd HH:mm:ss.fff} [{level}] [{component}] {message}");
+        if (ex != null)
+        {
+            sb.AppendLine();
+            sb.Append($"Exception: {ex.GetType().Name}: {ex.Message}");
+            if (!string.IsNullOrWhiteSpace(ex.StackTrace))
+            {
+                sb.AppendLine();
+                sb.Append(ex.StackTrace);
+            }
+
+            if (ex.InnerException != null)
+            {
+                sb.AppendLine();
+                sb.Append($"InnerException: {ex.InnerException.GetType().Name}: {ex.InnerException.Message}");
+            }
+        }
+
+        sb.AppendLine();
+        return sb.ToString();
+    }
+}