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-static-assets.js

@@ -0,0 +1,46 @@
+'use strict';
+
+var fs = require('fs');
+var path = require('path');
+var root = path.resolve(__dirname, '..');
+var passed = 0;
+
+function read(file) { return fs.readFileSync(path.join(root, file), 'utf8'); }
+function json(file) { return JSON.parse(read(file)); }
+function assert(condition, message) {
+  if (!condition) { throw new Error(message); }
+  passed++;
+}
+
+['config/config.json', 'config/package-solution.json', 'config/serve.json',
+  'src/extensions/customBranding/CustomBrandingApplicationCustomizer.manifest.json',
+  'examples/custom-branding.example.json']
+  .forEach(function (file) { json(file); passed++; });
+
+var packageJson = json('package.json');
+var solution = json('config/package-solution.json').solution;
+var serveText = read('config/serve.json').toLowerCase();
+var appSource = read('src/extensions/customBranding/CustomBrandingApplicationCustomizer.ts');
+var rendererSource = read('src/extensions/customBranding/BrandingDomRenderer.ts');
+var classicSource = read('classic/custom-branding-classic.js');
+
+assert(packageJson.version === '3.0.0', 'package.json hat nicht Version 3.0.0.');
+assert(solution.version === '3.0.0.0', 'Solution-Version ist inkonsistent.');
+assert(solution.skipFeatureDeployment === true, 'Tenantweite Bereitstellung ist nicht aktiviert.');
+assert(!solution.features, 'Die alte web-scoped Feature-Registrierung ist noch vorhanden.');
+assert(!fs.existsSync(path.join(root, 'sharepoint/assets/elements.xml')), 'elements.xml muss entfernt sein.');
+assert(serveText.indexOf('onclick') < 0, 'serve.json enthaelt ein Event-Attribut.');
+assert(serveText.indexOf('placeholdertop') >= 0, 'serve.json verwendet nicht das echte Schema.');
+assert(rendererSource.indexOf('innerHTML') < 0 && appSource.indexOf('innerHTML') < 0, 'Der moderne Renderer darf innerHTML nicht verwenden.');
+assert(classicSource.indexOf('innerHTML') < 0, 'Die Classic-Runtime darf innerHTML nicht verwenden.');
+assert(appSource.indexOf('changedEvent.remove') >= 0, 'Lifecycle-Cleanup fuer changedEvent fehlt.');
+assert(appSource.indexOf('_onTopPlaceholderDisposed') >= 0 && appSource.indexOf('_onBottomPlaceholderDisposed') >= 0,
+  'Top- und Bottom-Placeholder brauchen unabhaengige Dispose-Handler.');
+assert(appSource.indexOf("'CustomBrandingTopHost'") >= 0 && appSource.indexOf("'CustomBrandingBottomHost'") >= 0,
+  'Stabile Branding-Hosts fehlen.');
+assert(packageJson.scripts.package.indexOf('npm test') === 0, 'Paketierung muss mit Tests beginnen.');
+assert(!packageJson.dependencies['@microsoft/sp-dialog'], 'Nicht verwendete Dialog-Abhaengigkeit ist noch vorhanden.');
+
+new Function(classicSource);
+passed++;
+console.log('Statische Assets: ' + passed + ' Pruefungen erfolgreich.');