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/lib.rs

@@ -1,18 +1,31 @@
-use tauri::{RunEvent, WindowEvent, Emitter, Manager};
+use tauri::{Manager, RunEvent, WindowEvent, Emitter};
 
 mod utils;
 mod commands;
+mod state;
 
 use commands::{
-    start_backend,
-    check_backend_health,
-    get_opened_files,
-    clear_opened_files,
-    cleanup_backend,
     add_opened_file,
+    check_backend_health,
+    cleanup_backend,
+    clear_auth_token,
+    clear_opened_files,
+    clear_user_info,
     is_default_pdf_handler,
+    get_auth_token,
+    get_backend_port,
+    get_connection_config,
+    get_opened_files,
+    get_user_info,
+    is_first_launch,
+    login,
+    save_auth_token,
+    save_user_info,
+    set_connection_mode,
     set_as_default_pdf_handler,
+    start_backend,
 };
+use state::connection_state::AppConnectionState;
 use utils::{add_log, get_tauri_logs};
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
@@ -20,6 +33,9 @@ pub fn run() {
   tauri::Builder::default()
     .plugin(tauri_plugin_shell::init())
     .plugin(tauri_plugin_fs::init())
+    .plugin(tauri_plugin_http::init())
+    .plugin(tauri_plugin_store::Builder::new().build())
+    .manage(AppConnectionState::default())
     .plugin(tauri_plugin_single_instance::init(|app, args, _cwd| {
       // This callback runs when a second instance tries to start
       add_log(format!("📂 Second instance detected with args: {:?}", args));
@@ -60,12 +76,23 @@ pub fn run() {
     })
     .invoke_handler(tauri::generate_handler![
       start_backend,
-      check_backend_health,
+      get_backend_port,
       get_opened_files,
       clear_opened_files,
       get_tauri_logs,
+      get_connection_config,
+      set_connection_mode,
       is_default_pdf_handler,
       set_as_default_pdf_handler,
+      is_first_launch,
+      check_backend_health,
+      login,
+      save_auth_token,
+      get_auth_token,
+      clear_auth_token,
+      save_user_info,
+      get_user_info,
+      clear_user_info,
     ])
     .build(tauri::generate_context!())
     .expect("error while building tauri application")