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/package-lock.json

@@ -43,6 +43,7 @@
         "@tanstack/react-virtual": "^3.13.12",
         "@tauri-apps/api": "^2.5.0",
         "@tauri-apps/plugin-fs": "^2.4.0",
+        "@tauri-apps/plugin-http": "^2.5.4",
         "autoprefixer": "^10.4.21",
         "axios": "^1.12.2",
         "globals": "^16.4.0",
@@ -3887,6 +3888,15 @@
         "@tauri-apps/api": "^2.8.0"
       }
     },
+    "node_modules/@tauri-apps/plugin-http": {
+      "version": "2.5.4",
+      "resolved": "https://registry.npmjs.org/@tauri-apps/plugin-http/-/plugin-http-2.5.4.tgz",
+      "integrity": "sha512-/i4U/9za3mrytTgfRn5RHneKubZE/dwRmshYwyMvNRlkWjvu1m4Ma72kcbVJMZFGXpkbl+qLyWMGrihtWB76Zg==",
+      "license": "MIT OR Apache-2.0",
+      "dependencies": {
+        "@tauri-apps/api": "^2.8.0"
+      }
+    },
     "node_modules/@testing-library/dom": {
       "version": "10.4.1",
       "resolved": "https://registry.npmjs.org/@testing-library/dom/-/dom-10.4.1.tgz",