feat: Add custom branding functionality with CSS and JSON configuration

- Introduced custom branding CSS styles in `custom-branding.css`.
- Created example JSON configuration for custom branding in `custom-branding.example.json`.
- Implemented branding configuration logic in `BrandingConfig.ts` to normalize and validate branding settings.
- Developed CSS loader to manage loading and unloading of custom stylesheets in `BrandingCssLoader.ts`.
- Added DOM rendering capabilities for branding elements in `BrandingDomRenderer.ts`.
- Defined types and interfaces for branding elements and configurations in `BrandingTypes.ts`.
- Included localization support for German in `de-de.js`.
- Added unit tests for branding configuration, CSS loader, and DOM renderer.
- Validated project structure and static assets with new validation scripts.

classic/classic-deployment.md

@@ -0,0 +1,30 @@
+# CustomBranding für klassische SharePoint-Seiten
+
+Die Classic-Runtime liest dieselbe zentrale `SPSite.UserCustomAction` wie der moderne Application Customizer. Sie rendert das abgesicherte Konfigurationsschema ohne SPFx-Abhängigkeit und lädt die freigegebenen Stylesheets.
+
+## Bereitstellung
+
+1. `npm run build:classic` ausführen.
+2. `classic/dist/custom-branding-classic.js` beispielsweise nach `/SiteAssets/custom-branding/` hochladen.
+3. Den site-scoped ScriptLink zusammen mit der zentralen Application-Customizer-Action registrieren:
+
+```powershell
+.\deployment\add-custombranding.ps1 `
+  -SiteUrl 'http://clshp001/sites/portal' `
+  -ClassicScriptUrl '~sitecollection/SiteAssets/custom-branding/custom-branding-classic.js'
+```
+
+Der ScriptLink gilt für Root Web, vorhandene Subwebs und später angelegte Subwebs der Site Collection. Zum Entfernen:
+
+```powershell
+.\deployment\add-custombranding.ps1 `
+  -SiteUrl 'http://clshp001/sites/portal' `
+  -RemoveClassicScriptLink
+```
+
+## Diagnose
+
+```javascript
+typeof window.CustomBrandingClassic
+window.CustomBrandingClassic.reload()
+```