feat: add Microsoft Clarity analytics provider

Add Microsoft Clarity as a fourth analytics provider option alongside
Umami, Google Analytics, and Custom. Clarity provides free heatmaps,
session recordings, and user analytics.

- admin.types.ts: Add 'clarity' to AnalyticsProvider union type
- api/admin/site/route.ts: Add 'clarity' to validation
- admin/page.tsx: Add Clarity option to dropdown + Project ID input UI
- layout.tsx: Inject Clarity tracking script with project ID

src/app/api/admin/site/route.ts

@@ -144,7 +144,7 @@ export async function POST(request: NextRequest) {
       OIDCButtonText?: string;
       OIDCMinTrustLevel?: number;
       AnalyticsEnabled?: boolean;
-      AnalyticsProvider?: 'umami' | 'google' | 'custom';
+      AnalyticsProvider?: 'umami' | 'google' | 'clarity' | 'custom';
       AnalyticsScriptUrl?: string;
       AnalyticsWebsiteId?: string;
       AnalyticsCustomScript?: string;
@@ -239,6 +239,7 @@ export async function POST(request: NextRequest) {
       (AnalyticsProvider !== undefined &&
         AnalyticsProvider !== 'umami' &&
         AnalyticsProvider !== 'google' &&
+        AnalyticsProvider !== 'clarity' &&
         AnalyticsProvider !== 'custom') ||
       (AnalyticsScriptUrl !== undefined && typeof AnalyticsScriptUrl !== 'string') ||
       (AnalyticsWebsiteId !== undefined && typeof AnalyticsWebsiteId !== 'string') ||