feat(config): add FileConfig model and JSON loader

src/ClaudeMailbox/Config/FileConfig.cs

@@ -0,0 +1,41 @@
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace ClaudeMailbox.Config;
+
+public sealed class FileConfig
+{
+    [JsonPropertyName("port")]
+    public int? Port { get; set; }
+
+    [JsonPropertyName("bind")]
+    public string? Bind { get; set; }
+
+    [JsonPropertyName("dbPath")]
+    public string? DbPath { get; set; }
+
+    private static readonly JsonSerializerOptions Options = new()
+    {
+        PropertyNameCaseInsensitive = true,
+    };
+
+    public static FileConfig Load(string? explicitPath, string? defaultPath)
+    {
+        if (!string.IsNullOrEmpty(explicitPath))
+        {
+            if (!File.Exists(explicitPath))
+                throw new FileNotFoundException($"Config file not found: {explicitPath}", explicitPath);
+            return Parse(File.ReadAllText(explicitPath));
+        }
+
+        if (!string.IsNullOrEmpty(defaultPath) && File.Exists(defaultPath))
+            return Parse(File.ReadAllText(defaultPath));
+
+        return new FileConfig();
+    }
+
+    private static FileConfig Parse(string json)
+    {
+        return JsonSerializer.Deserialize<FileConfig>(json, Options) ?? new FileConfig();
+    }
+}