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.

tests/validate-project.ps1

@@ -0,0 +1,23 @@
+$ErrorActionPreference = 'Stop'
+
+$root = Split-Path -Parent $PSScriptRoot
+$files = @(
+    (Join-Path $root 'deployment\add-custombranding.ps1'),
+    (Join-Path $root 'build-classic.ps1')
+)
+
+foreach ($file in $files) {
+    $tokens = $null
+    $errors = $null
+    [void][System.Management.Automation.Language.Parser]::ParseFile($file, [ref]$tokens, [ref]$errors)
+    if ($errors.Count -gt 0) {
+        throw "PowerShell-Syntaxfehler in $file`: $($errors[0].Message)"
+    }
+}
+
+$localizationFiles = Get-ChildItem (Join-Path $root 'src\extensions\customBranding\loc') -Filter '*.js'
+if ($localizationFiles.Count -lt 2) {
+    throw 'Deutsche und englische Lokalisierungsdateien werden erwartet.'
+}
+
+Write-Host "Projektvalidierung: $($files.Count) PowerShell- und $($localizationFiles.Count) Lokalisierungsdateien erfolgreich geprueft."