Document YouTube cookie auth and warn when the file is unusable

Cookies were already wired up but only mentioned in passing, and the
non-obvious parts were undocumented: yt-dlp has no username/password
support for YouTube, it rewrites the cookie file to persist rotated
cookies (so a read-only file expires early), and the export has to
happen in a private window that is logged out before closing.

Startup now reports whether the cookie file is usable, missing, or
read-only, and YTDLP_EXTRACTOR_ARGS is passed through for the cases
where YouTube blocks a server IP outright.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/config.ts

@@ -36,6 +36,8 @@ const schema = z.object({
 
   YTDLP_PATH: z.string().default("yt-dlp"),
   YTDLP_COOKIES: z.string().optional(),
+  /** Extra `--extractor-args` values, separated by ";" — e.g. youtube:player_client=default,web_safari */
+  YTDLP_EXTRACTOR_ARGS: z.string().optional(),
   LOCAL_MEDIA_DIR: z.string().optional(),
 
   DEFAULT_VOLUME: z.coerce.number().min(0).max(200).default(60),