Rewrite frontend as single self-contained HTML file — all CSS/JS inline, no external files to fail loading

dist/server/services/ytDlp.d.ts

@@ -0,0 +1,7 @@
+export interface YtDlpResult {
+    filePath: string;
+    filename: string;
+    title: string;
+}
+export declare function isYtDlpAvailable(): boolean;
+export declare function downloadVideo(url: string, outputDir: string): YtDlpResult;