Replace uses of atob and btoa with modern equivalents

### Description

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 dropping support for node 24 (and below) and updating the target library in tsconfig.

Initially raised in https://github.com/getodk/web-forms/pull/766#discussion_r3077824569

Search the repository for uses of atob and btoa, then inspect tsconfig and the Node support configuration. Replace the calls with the specified Uint8Array methods, update the target library, and verify that support for Node 24 and below is dropped consistently with the issue and linked pull request discussion.

Written by the indexing model from the issue text.