feat(config): wire /config/update + /config/update_challenge_config (closes family 4/4)

is_skip_gacha_effect, use_challenge_two_pick_premium_card, and
challenge_two_pick_sleeve_id all persist on ViewerInfo and round-trip
through /load/index.

The two challenge fields move from the global ChallengeConfig section to
ViewerInfo — they're viewer preferences, not server-wide policy. Premium
card defaults to 0; sleeve falls back to DefaultLoadoutConfig.SleeveId
(3000011) when unset. MatchContextBuilder's TK2 sleeve read switches to
the same viewer-scoped path.

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

SVSim.Database/Models/Config/ChallengeConfig.cs

@@ -3,9 +3,6 @@ namespace SVSim.Database.Models.Config;
 [ConfigSection("Challenge")]
 public class ChallengeConfig
 {
-    public bool UseTwoPickPremiumCard { get; set; }
-    public long TwoPickSleeveId { get; set; }
-
     // Wire-mirrored fields from format_info (ChallengeData.cs parser)
     public int LastCardPackSetId { get; set; }
     public string CardPoolName { get; set; } = "";

SVSim.Database/Models/ViewerInfo.cs

@@ -12,6 +12,9 @@ public class ViewerInfo
     public bool IsOfficialMarkDisplayed { get; set; }
     public bool IsFoilPreferred { get; set; }
     public bool IsPrizePreferred { get; set; }
+    public bool IsSkipGachaEffect { get; set; }
+    public bool UseChallengeTwoPickPremiumCard { get; set; }
+    public long ChallengeTwoPickSleeveId { get; set; }
 
     #region Navigation Properties