Add Portal Settings Web Part with styles, localization, and tests

- Implemented the PortalSettings Web Part with a new manifest and TypeScript file.
- Created SCSS styles for the Web Part, defining various UI elements and responsive design.
- Added localization support for English and German languages.
- Developed tests for Provider Registry and static assets validation.
- Included PowerShell script for project validation.

tests/validate-project.ps1

@@ -0,0 +1,26 @@
+$ErrorActionPreference = 'Stop'
+$root = Split-Path -Parent $PSScriptRoot
+
+$tokens = $null
+$errors = $null
+[void][System.Management.Automation.Language.Parser]::ParseFile(
+    (Join-Path $root 'deployment\Add-PortalSettingsPage.ps1'),
+    [ref]$tokens,
+    [ref]$errors
+)
+if ($errors.Count -gt 0) {
+    throw ('PowerShell-Syntaxfehler: ' + $errors[0].Message)
+}
+
+$jsonFiles = @(
+    'package.json',
+    'config\config.json',
+    'config\package-solution.json',
+    'config\serve.json',
+    'src\webparts\portalSettings\PortalSettingsWebPart.manifest.json'
+)
+foreach ($relativePath in $jsonFiles) {
+    Get-Content -Raw -LiteralPath (Join-Path $root $relativePath) | ConvertFrom-Json | Out-Null
+}
+
+Write-Host ("PortalSettings project validation passed: 1 PowerShell file and $($jsonFiles.Count) JSON files.")