Add native self-hosted instance connection to fluxer_desktop

Trimmed monorepo checkout (fluxer_desktop + packages/voice_engine_v2 +
tools/ci) with a "Connect to a Different Server" menu item and popout
that lets the desktop app switch to any self-hosted Fluxer instance,
plus fixes for well-known discovery on single-domain self-hosted
deployments and a false-positive ERR_ABORTED on same-origin client
redirects during the switch. Defaults to chat.fluxr.chat and uses an
isolated userData directory from the official build.

fluxer_desktop/native/audio-mix/Cargo.toml

@@ -0,0 +1,23 @@
+[package]
+name = "fluxer_audio_mix"
+version = "0.0.0"
+edition = "2024"
+license = "AGPL-3.0-or-later"
+publish = false
+
+[workspace]
+resolver = "2"
+
+[lib]
+crate-type = ["rlib"]
+
+[dependencies]
+rtrb = "0.3"
+fluxer_rt_thread = {path = "../rt-thread"}
+
+[dev-dependencies]
+criterion = "0.8"
+
+[[bench]]
+name = "mix"
+harness = false