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/vite.config.ts

@@ -31,24 +31,25 @@ export default defineConfig(({ mode }) => {
         // tell vite to ignore watching `src-tauri`
         ignored: ['**/src-tauri/**'],
       },
-    proxy: {
-      '/api': {
-        target: 'http://localhost:8080',
-        changeOrigin: true,
-        secure: false,
-        xfwd: true,
-      },
-      '/oauth2': {
-        target: 'http://localhost:8080',
-        changeOrigin: true,
-        secure: false,
-        xfwd: true,
-      },
-      '/login/oauth2': {
-        target: 'http://localhost:8080',
-        changeOrigin: true,
-        secure: false,
-        xfwd: true,
+      // Only use proxy in web mode - Tauri handles backend connections directly
+      proxy: isDesktopMode ? undefined : {
+        '/api': {
+          target: 'http://localhost:8080',
+          changeOrigin: true,
+          secure: false,
+          xfwd: true,
+        },
+        '/oauth2': {
+          target: 'http://localhost:8080',
+          changeOrigin: true,
+          secure: false,
+          xfwd: true,
+        },
+        '/login/oauth2': {
+          target: 'http://localhost:8080',
+          changeOrigin: true,
+          secure: false,
+          xfwd: true,
         },
       },
     },