fix: adapt code to modernized libraries

- Migrate validation.ts from class-transformer/class-validator to Zod
- Apply Biome auto-fixes (import sorting, nodejs protocol)
- Fix duplicate type identifier in validation.ts

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/muxer-aup3.ts

@@ -1,6 +1,6 @@
+import fs from "node:fs";
+import path from "node:path";
 import ffmpeg from "fluent-ffmpeg";
-import fs from "fs";
-import path from "path";
 
 const recordingsDir = process.env.RECORDINGS_DIR ?? "./recordings";