feat: implement input stream support for web backends

[roderickvd]

Implements missing microphone/input support for WebAudio and Emscripten backends. Previously, calling any input-related methods would return empty results or panic.

Implementation

Uses navigator.mediaDevices.getUserMedia() to request microphone access, then creates a MediaStreamAudioSourceNode connected to a ScriptProcessorNode for audio capture. The async getUserMedia Promise is handled via wasm_bindgen_futures::spawn_local(), allowing the synchronous API to return immediately while permission is requested in the background.

Status

This is very preliminary and mostly untested code. Please test and report back here.