practice/deck_list returns the same wire shape as /deck/info (the client parses
both via DeckGroupListData), but only ever sent user decks — so a fresh account
saw no default decks and couldn't start a practice match.
Extract the /deck/info hydration into a shared IDeckListBuilder used by
/deck/info, /deck/my_list, and /practice/deck_list. Practice passes
padEmptySlots:false (deck *select*, not builder) — matches the prod practice
capture, which returns real decks unpadded plus the 8 per-class default decks
and per-class leader-skin settings. Retire the near-duplicate
PracticeDeckListResponse DTO in favor of the shared DeckListResponse.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
