feat(ui): add decision-log reader and overlay components

ui/JevOverlay/Core/OverlayConfig.cs

@@ -0,0 +1,21 @@
+using System.Text.Json;
+
+namespace JevOverlay.Core;
+
+public static class OverlayConfig
+{
+    public const string FileName = "JevOverlay.config.json";
+
+    public static string LoadLogPath(string modDirectory)
+    {
+        using var document = JsonDocument.Parse(File.ReadAllText(Path.Combine(modDirectory, FileName)));
+        var root = document.RootElement;
+        if (root.ValueKind != JsonValueKind.Object ||
+            !root.TryGetProperty("log_path", out var value) ||
+            value.ValueKind != JsonValueKind.String ||
+            string.IsNullOrWhiteSpace(value.GetString()) ||
+            !Path.IsPathFullyQualified(value.GetString()!))
+            throw new FormatException("log_path must be an absolute file path.");
+        return Path.GetFullPath(value.GetString()!);
+    }
+}