Add SaaS frontend code (#5879)

# Description of Changes
Adds the code for the SaaS frontend as proprietary code to the OSS repo.
This version of the code is adapted from 22/1/2026, which was the last
SaaS version based on the 'V2' design. This will move us closer to being
able to have the OSS products understand whether the user has a SaaS
account, and provide the correct UI in those cases.

frontend/src/desktop/LICENSE

@@ -0,0 +1,51 @@
+Stirling PDF User License
+
+Copyright (c) 2025 Stirling PDF Inc.
+
+License Scope & Usage Rights
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+Production use of the Stirling PDF Software is only permitted with a valid Stirling PDF User License.
+
+For purposes of this license, “the Software” refers to the Stirling PDF application and any associated documentation files
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+processes unless you have agreed to, and remain in compliance with, the Stirling PDF Subscription Terms of Service
+(https://www.stirlingpdf.com/terms) or another valid agreement with Stirling PDF, and hold an active User License subscription
+covering the appropriate number of licensed users.
+
+Trial and Minimal Use
+
+You may use the Software without a paid subscription for the sole purposes of internal trial, evaluation, or minimal use, provided that:
+* Use is limited to the capabilities and restrictions defined by the Software itself;
+* You do not copy, distribute, sublicense, reverse-engineer, or use the Software in client-facing or commercial contexts.
+
+Continued use beyond this scope requires a valid Stirling PDF User License.
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+Modifications and Derivative Works
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+You may modify the Software only for development or internal testing purposes. Any such modifications or derivative works:
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+* May not be deployed in production environments without a valid User License;
+* May not be distributed or sublicensed;
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frontend/src/desktop/components/shared/config/configNavSections.tsx

@@ -8,6 +8,8 @@ import { SaaSTeamsSection } from '@app/components/shared/config/configSections/S
 import { connectionModeService } from '@app/services/connectionModeService';
 import { authService } from '@app/services/authService';
 
+export type { ConfigNavSection, ConfigNavItem } from '@core/components/shared/config/configNavSections';
+
 /**
  * Hook version of desktop config nav sections with proper i18n support
  */

frontend/src/desktop/tsconfig.json

@@ -0,0 +1,27 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "baseUrl": "../../",
+    "paths": {
+      "@app/*": [
+        "src/desktop/*",
+        "src/proprietary/*",
+        "src/core/*"
+      ],
+      "@proprietary/*": [
+        "src/proprietary/*"
+      ],
+      "@core/*": [
+        "src/core/*"
+      ]
+    }
+  },
+  "include": [
+    "../global.d.ts",
+    "../*.js",
+    "../*.ts",
+    "../*.tsx",
+    "../core/setupTests.ts",
+    "."
+  ]
+}