feat: add scoped theme builder system with per-route, per-module, and multi-site support

- Add theme_scopes and theme_scope_values database tables for scoped themes
- Implement theme resolver engine with inheritance: global > site > module > route
- Add module detection for 20+ routes (shop, guilds, radio, news, etc.)
- Create admin UI at /admin/theme-builder with scope tree and color editor
- Add ScopedThemeVars component for injecting scoped CSS via data-attributes
- Add ThemeScopeDetector client component for runtime module/route detection
- Add site-resolver for multi-site domain detection
- Add /api/themes/export endpoint (JSON, CSS, variables formats)
- Add /api/themes/export/embed.js for external integration widget
- Add server actions for full CRUD on scopes and theme values
- Add admin nav link and EN/NL translations

src/lib/site-resolver.ts

@@ -0,0 +1,34 @@
+import { headers } from "next/headers";
+
+/**
+ * Resolve the current site domain from request headers.
+ * Used by the theme resolver to apply site-scoped themes.
+ */
+export async function resolveSiteDomain(): Promise<string | null> {
+	const h = await headers();
+	// Next.js sets host in the Host header
+	const host = h.get("host") ?? h.get("x-forwarded-host");
+	if (!host) return null;
+	// Strip port if present
+	return host.split(":")[0];
+}
+
+/**
+ * Build the full theme context from the current request.
+ * Combines site domain + route path + module detection.
+ */
+export async function buildThemeContext(pathname: string): Promise<{
+	siteDomain: string | null;
+	routePath: string;
+	moduleId: string | null;
+}> {
+	const siteDomain = await resolveSiteDomain();
+	const { detectModule } = await import("@/lib/theme-modules");
+	const moduleId = detectModule(pathname);
+
+	return {
+		siteDomain,
+		routePath: pathname,
+		moduleId,
+	};
+}