Replace custom code with built-in browser APIs

[sosnovsky]

There are some code parts using our custom implementations which can be replaced with modern built-in browser APIs:

• fromUtfStr manually handles Unicode and can be replaced with TextEncoder which is widely supported (https://developer.mozilla.org/en-US/docs/Web/API/TextEncoder)
• Promise in readAttachmentDataAsUint8 handles only onload, an asynchronous read failure leaves it pending indefinitely. This can become return new Uint8Array(await this.attachedFiles[uploadFileId].arrayBuffer()) which removes callback plumbing and propagates failures naturally (https://developer.mozilla.org/en-US/docs/Web/API/Blob/arrayBuffer)
• use native Uint8Array.fromBase64() and toBase64() APIs which appeared in Chrome 140 instead of base64decode and base64encode