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.json

@@ -26,8 +26,6 @@
     "@embedpdf/plugin-viewport": "^1.4.1",
     "@embedpdf/plugin-zoom": "^1.4.1",
     "@emotion/react": "^11.14.0",
-    "@tauri-apps/api": "^2.5.0",
-    "@tauri-apps/plugin-fs": "^2.4.0",
     "@emotion/styled": "^11.14.1",
     "@iconify/react": "^6.0.2",
     "@mantine/core": "^8.3.1",
@@ -39,6 +37,9 @@
     "@reactour/tour": "^3.8.0",
     "@tailwindcss/postcss": "^4.1.13",
     "@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",
@@ -110,11 +111,11 @@
     ]
   },
   "devDependencies": {
-    "@tauri-apps/cli": "^2.5.0",
     "@eslint/js": "^9.36.0",
     "@iconify-json/material-symbols": "^1.2.37",
     "@iconify/utils": "^3.0.2",
     "@playwright/test": "^1.55.0",
+    "@tauri-apps/cli": "^2.5.0",
     "@testing-library/dom": "^10.4.1",
     "@testing-library/jest-dom": "^6.8.0",
     "@testing-library/react": "^16.3.0",