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/shared/config/configNavSections.tsx

@@ -0,0 +1,30 @@
+import { createConfigNavSections as createProprietaryConfigNavSections } from '@proprietary/components/shared/config/configNavSections';
+import { ConfigNavSection } from '@core/components/shared/config/configNavSections';
+import { ConnectionSettings } from '@app/components/ConnectionSettings';
+
+/**
+ * Desktop extension of createConfigNavSections that adds connection settings
+ */
+export const createConfigNavSections = (
+  isAdmin: boolean = false,
+  runningEE: boolean = false,
+  loginEnabled: boolean = false
+): ConfigNavSection[] => {
+  // Get the proprietary sections (includes core Preferences + admin sections)
+  const sections = createProprietaryConfigNavSections(isAdmin, runningEE, loginEnabled);
+
+  // Add Connection section at the beginning (after Preferences)
+  sections.splice(1, 0, {
+    title: 'Connection',
+    items: [
+      {
+        key: 'connectionMode',
+        label: 'Connection Mode',
+        icon: 'cloud-rounded',
+        component: <ConnectionSettings />,
+      },
+    ],
+  });
+
+  return sections;
+};

frontend/src/desktop/components/shared/config/types.ts

@@ -0,0 +1,8 @@
+import { VALID_NAV_KEYS as CORE_NAV_KEYS } from '@core/components/shared/config/types';
+
+export const VALID_NAV_KEYS = [
+  ...CORE_NAV_KEYS,
+  'connectionMode',
+] as const;
+
+export type NavKey = typeof VALID_NAV_KEYS[number];