Add NLog file logging configuration and error handling

Integrate NLog for structured file-based logging by adding configuration to appsettings.json and initializing NLog in Program.cs. Wrap application startup in a try-catch block to log unhandled exceptions. No changes to core application logic.

appsettings.json

@@ -11,5 +11,48 @@
       "Protocols": "Http1"
     }
   },
-  "EnableSwagger": true
+  "EnableSwagger": true,
+  "NLog": {
+    "throwConfigExceptions": true,
+    "variables": {
+      "logDirectory": "E:\\LogFiles\\Digital Data\\FakeNTLMServer",
+      "logFileNamePrefix": "${shortdate}-FakeNTLMServer"
+    },
+    "targets": {
+      "infoLogs": {
+        "type": "File",
+        "fileName": "${logDirectory}\\${logFileNamePrefix}-Info.log",
+        "maxArchiveDays": 30
+      },
+      "errorLogs": {
+        "type": "File",
+        "fileName": "${logDirectory}\\${logFileNamePrefix}-Error.log",
+        "maxArchiveDays": 30
+      },
+      "criticalLogs": {
+        "type": "File",
+        "fileName": "${logDirectory}\\${logFileNamePrefix}-Critical.log",
+        "maxArchiveDays": 30
+      }
+    },
+    // Trace, Debug, Info, Warn, Error and *Fatal*
+    "rules": [
+      {
+        "logger": "*",
+        "minLevel": "Info",
+        "maxLevel": "Warn",
+        "writeTo": "infoLogs"
+      },
+      {
+        "logger": "*",
+        "level": "Error",
+        "writeTo": "errorLogs"
+      },
+      {
+        "logger": "*",
+        "level": "Fatal",
+        "writeTo": "criticalLogs"
+      }
+    ]
+  }
 }