### Description <!-- Provide a concise description of the task. What specific work needs to be done? --> Replace usage of [atob](https://developer.mozilla.org/en-US/docs/Web/API/Window/atob) with [Uint8Array.fromBase64()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/fromBase64), and [btoa](https://developer.mozilla.org/en-US/docs/Web/API/Window/btoa) with [Uint8Array.toBase64()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Uint8Array/toBase64). Requires updating the target library in tsconfig. Initially raised in https://github.com/getodk/web-forms/pull/766#discussion_r3077824569
Description
Replace usage of atob with Uint8Array.fromBase64(), and btoa with Uint8Array.toBase64().
Requires updating the target library in tsconfig.
Initially raised in #766 (comment)