Fall back to spec retention defaults before the global default

GetRetentionDays previously dropped straight from a missing user
override to RetentionDefaultDays, so every channel showed 30 days
in the UI even though the spec lists 365 for Tells and 90 for own-
conversation channels. Insert a middle layer: user override → spec
default → global default. The retention sweep now seeds its policy
from PrivacyDefaults.DefaultRetentionDays first and lets explicit
user overrides win on top, and the per-channel UI tags each row as
[override], [spec], or [global] so the source of the value is
visible without guessing.

ChatTwo/Configuration.cs

@@ -66,6 +66,8 @@ public class Configuration : IPluginConfiguration
     {
         if (RetentionPerChannelDays.TryGetValue(type, out var userOverride))
             return userOverride;
+        if (Privacy.PrivacyDefaults.DefaultRetentionDays.TryGetValue(type, out var specDefault))
+            return specDefault;
         return RetentionDefaultDays;
     }