feat(config): bump schema to v20 with style-refactor visibility fields

Adds MainWindowOpen, SettingsWindowOpen, MaxParallelPopouts (channel popout
pool size), TellAutoOpenMode (Off/Sidebar/TopTab/Popout) and
SidebarAutoSwitchThresholdPx. Migration is additive — field initializers
fill defaults for v19 configs, the Plugin.cs schema gate bumps the version
stamp after load. UpdateFrom gets explicit sync statements for all five so
settings-save edits do not drop them.

HellionChat/Plugin.cs

@@ -200,11 +200,12 @@ public sealed class Plugin : IAsyncDalamudPlugin
         // do not touch either static, so the brief null-window is safe.
 
         // Schema gate: v1.4.x+ requires config v16+. Users on older schemas
-        // must install v1.4.2 first to run the migration chain. v19 adds the
+        // must install v1.4.2 first to run the migration chain. v19 added the
         // top-level CustomSoundVolume, WindowOpacityInactive, WorldSuffixMode
-        // and NameFormMode fields — all additive with defaults, so v16-v18
-        // configs load cleanly and get their Version stamp bumped after the
-        // gate.
+        // and NameFormMode fields; v20 adds MainWindowOpen, SettingsWindowOpen,
+        // MaxParallelPopouts, TellAutoOpenMode and SidebarAutoSwitchThresholdPx
+        // — all additive with defaults, so v16-v19 configs load cleanly and
+        // get their Version stamp bumped after the gate.
         if (Config.Version < 16)
         {
             throw new InvalidOperationException(
@@ -212,7 +213,7 @@ public sealed class Plugin : IAsyncDalamudPlugin
                     + "Please install v1.4.2 first to migrate the configuration, then upgrade to v1.4.10."
             );
         }
-        Config.Version = 19;
+        Config.Version = 20;
 
         // Unpinned TempTabs are session-only and dropped on every load. Pinned
         // TempTabs survive reload — Jin's tester feedback (v1.4.7).