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/tsconfig.desktop.json

@@ -13,6 +13,10 @@
     }
   },
   "exclude": [
+    "src/core/**/*.test.ts*",
+    "src/core/**/*.spec.ts*",
+    "src/proprietary/**/*.test.ts*",
+    "src/proprietary/**/*.spec.ts*",
     "node_modules"
   ]
 }