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/src/state/connection_state.rs

@@ -0,0 +1,45 @@
+use serde::{Deserialize, Serialize};
+use std::sync::Mutex;
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(rename_all = "lowercase")]
+pub enum ConnectionMode {
+    Offline,
+    Server,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
+#[serde(rename_all = "lowercase")]
+pub enum ServerType {
+    SaaS,
+    SelfHosted,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ServerConfig {
+    pub url: String,
+    pub server_type: ServerType,
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ConnectionState {
+    pub mode: ConnectionMode,
+    pub server_config: Option<ServerConfig>,
+}
+
+impl Default for ConnectionState {
+    fn default() -> Self {
+        Self {
+            mode: ConnectionMode::Offline,
+            server_config: None,
+        }
+    }
+}
+
+pub struct AppConnectionState(pub Mutex<ConnectionState>);
+
+impl Default for AppConnectionState {
+    fn default() -> Self {
+        Self(Mutex::new(ConnectionState::default()))
+    }
+}

frontend/src-tauri/src/state/mod.rs

@@ -0,0 +1 @@
+pub mod connection_state;