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-tauri/capabilities/default.json

@@ -7,9 +7,18 @@
   ],
   "permissions": [
     "core:default",
-    { 
-    "identifier": "fs:allow-read-file",
-    "allow": [{ "path": "**" }]
-   }
+    "http:default",
+    {
+      "identifier": "http:allow-fetch",
+      "allow": [
+        { "url": "http://localhost:*" },
+        { "url": "http://127.0.0.1:*" },
+        { "url": "https://*" }
+      ]
+    },
+    {
+      "identifier": "fs:allow-read-file",
+      "allow": [{ "path": "**" }]
+    }
   ]
 }