fix: replace deprecated glib::MainContext::channel with async_channel

[ptcodes]

The build was failing due to a couple of issues and I started looking into them. This pull request addresses the first of them on deprecation warnings on glib::MainContext::channel being treated as errors in CI.

The fix does the following:

• Replaces all uses of the deprecated glib::MainContext::channel, glib::Sender and glib::Receiver with async_channel::unbounded() across actions.rs, cli_handler.rs, pages/mod.rs, pages/dns.rs and pages/tweaks.rs
• Migrates receiver handlers from rx.attach() to glib::MainContext::default().spawn_local() with an async while let receive loop
• Uses send_blocking() in background threads and try_send() in sync GTK main-thread callbacks.