Allow desktop app to connect to selfhosted servers (#4902)

# Description of Changes
Changes the desktop app to allow connections to self-hosted servers on
first startup. This was quite involved and hit loads of CORS issues all
through the stack, but I think it's working now. This also changes the
bundled backend to spawn on an OS-decided port rather than always
spawning on `8080`, which means that the user can have other things
running on port `8080` now and the app will still work fine. There were
quite a few places that needed to be updated to decouple the app from
explicitly using `8080` and I was originally going to split those
changes out into another PR (#4939), but I couldn't get it working
independently in the time I had, so the diff here is just going to be
complex and contian two distinct changes - sorry 🙁

frontend/src-tauri/Cargo.toml

@@ -28,7 +28,10 @@ tauri = { version = "2.9.0", features = [ "devtools"] }
 tauri-plugin-log = "2.0.0-rc"
 tauri-plugin-shell = "2.1.0"
 tauri-plugin-fs = "2.4.4"
+tauri-plugin-http = "2.4.4"
 tauri-plugin-single-instance = "2.0.1"
+tauri-plugin-store = "2.1.0"
+keyring = "3.6.1"
 tokio = { version = "1.0", features = ["time"] }
 reqwest = { version = "0.11", features = ["json"] }