feat(practice): serve default/trial/leader-skin lists on practice/deck_list

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>

SVSim.EmulatedEntrypoint/Program.cs

@@ -100,6 +100,7 @@ public class Program
         builder.Services.AddScoped<IStoryMasterRepository, StoryMasterRepository>();
         builder.Services.AddScoped<IViewerStoryProgressRepository, ViewerStoryProgressRepository>();
         builder.Services.AddScoped<IStoryService, StoryService>();
+        builder.Services.AddScoped<IDeckListBuilder, DeckListBuilder>();
         builder.Services.AddSingleton<IRandom, SystemRandom>();
         builder.Services.AddSingleton<PuzzleMissionEvaluator>();