feat: implement per-app recording

[RangeyRover]

"Hi mate, I extended the recorder to support Per-Application isolated recording via WASAPI loopback (so you can record Discord without recording game sound). Here is the code if you want to merge it."

[lukmay]

Hi @RangeyRover, thanks for this. Per-app recording via WASAPI loopback is a cool addition!

Before I dive into the review, a few things I want to clarify with you:

1. UX: how does a user select the target app? Is it a new dropdown in the settings, does it replace "System Audio" mode, or does it sit alongside it? A screenshot or short clip of the new UI would help a lot.

2. Fallback: what happens on Windows versions where the per-app loopback API isn't available (pre Windows 10 2004, I think)? Does it fail gracefully or crash?

3. Testing: what did you test this against? Just Discord, or multiple apps? Any weirdness with apps that change their audio session (e.g. browsers switching tabs)?

I'll also want to test it locally before merging, so give me a few days on that.

Also: I just added a CONTRIBUTING.md to the repo, nothing you did wrong, just putting it in place for future PRs.