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/desktop/components/SetupWizard/SetupWizard.css

@@ -0,0 +1,20 @@
+.setup-container {
+  position: relative;
+  min-height: 100vh;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  background: linear-gradient(135deg, #f5f5f5 0%, #e8e8e8 100%);
+  padding: 2rem;
+}
+
+.setup-wrapper {
+  width: 100%;
+  max-width: 600px;
+}
+
+.setup-card {
+  background-color: white;
+  box-shadow: 0 20px 60px rgba(0, 0, 0, 0.12);
+}