chore(types.d.ts): add missing platform prop to CustomGlobals

as used e.g. in dekstop.js – supplied via ejs (and then rendered via `./src/views/partials/windowGlobal`

I left it as optional however, because e.g. it does not seem to exist on *all* pages

src/public/app/types.d.ts

@@ -43,6 +43,7 @@ interface CustomGlobals {
     appCssNoteIds: string[];
     triliumVersion: string;
     TRILIUM_SAFE_MODE: boolean;
+    platform?: typeof process.platform;
 }
 
 type RequireMethod = (moduleName: string) => any;