add better typing to Electron.tsx

src/@types/global.d.ts

@@ -1,8 +1,13 @@
 // Defined by webpack on startup or compilation
-declare let __COMMIT_HASH__: string;
+declare const __COMMIT_HASH__: string;
 
 // When using file-loader, we'll get a path to the resource
 declare module "*.png" {
   const value: string;
   export default value;
 }
+
+// Achievements communicated back to Electron shell for Steam.
+declare interface Document {
+  achievements: string[];
+}