feat: migrate to API token auth, remove legacy parser

- Add --token parameter (takes priority over --cookie)
- Deprecate --cookie and --useragent with warning messages
- Remove --legacy flag and legacy HTML search parser
- Migrate --artist search to use v2 API query syntax (artist:slug)
- Bump version to 2.0.6

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

doujinshi_dl/command.py

@@ -82,17 +82,15 @@ def main():
             options.keyword,
             sorting=options.sorting,
             page=page_list,
-            legacy=options.legacy,
             is_page_all=options.page_all,
         )
 
     elif options.artist:
         doujinshis = parser.search(
-            options.artist,
+            f'artist:{options.artist}',
             sorting=options.sorting,
             page=page_list,
             is_page_all=options.page_all,
-            type_='ARTIST',
         )
 
     elif not doujinshi_ids: