feat(window): restore the title-bar, hide-button, and 24-hour-clock toggles

Re-wires four 1.5.6 settings that survived the v1.6.0 rewrite as dormant config
fields but lost their UI + consumers:
- ShowTitleBar / ShowPopOutTitleBar: gate ImGuiWindowFlags.NoTitleBar on the main
  window (ResolveFlags) and pop-out windows (new PreDraw). Inverted logic matches
  1.5.6 (flag set only when the toggle is off).
- ShowHideButton: gate the input-bar hide button on the toggle.
- Use24HourClock: add the toggle (MessageList already consumes the field).
New 'Window style' section in WindowTab; Use24HourClock in ChatTab display modes.
MainWindowFlagsStep extended with the NoTitleBar fresh-base contract.

HellionChat/Ui/Windows/MainWindow.cs

@@ -77,19 +77,21 @@ internal sealed class MainWindow : Window
     internal float ResolveBgAlpha(bool isFocused) =>
         isFocused ? Plugin.Config.WindowOpacity : Plugin.Config.WindowOpacityInactive;
 
-    // B1-2: rebuild flags from a fresh base every frame so toggling CanMove/
-    // CanResize back on actually CLEARS NoMove/NoResize (not accumulating).
-    // Move/resize toggle logic as 1.5.6 (ChatLogWindow.PreOpenCheck
-    // 1d3b429:703-707); base flags = today's MainWindow set (NoScrollbar|
-    // NoScrollWithMouse — the message list owns its own scroll; 1.5.6's
-    // NoFocusOnAppearing/NoTitleBar are deliberately not restored).
-    internal static ImGuiWindowFlags ResolveFlags(bool canMove, bool canResize)
+    // B1-2 / P7: rebuild flags from a fresh base every frame so toggling
+    // CanMove/CanResize/ShowTitleBar back on actually CLEARS NoMove/NoResize/
+    // NoTitleBar (not accumulating). Move/resize/title-bar logic as 1.5.6
+    // (ChatLogWindow.PreOpenCheck 1d3b429:703-710); base flags = today's
+    // MainWindow set (NoScrollbar|NoScrollWithMouse — the message list owns its
+    // own scroll; 1.5.6's NoFocusOnAppearing is deliberately not restored).
+    internal static ImGuiWindowFlags ResolveFlags(bool canMove, bool canResize, bool showTitleBar)
     {
         var flags = ImGuiWindowFlags.NoScrollbar | ImGuiWindowFlags.NoScrollWithMouse;
         if (!canMove)
             flags |= ImGuiWindowFlags.NoMove;
         if (!canResize)
             flags |= ImGuiWindowFlags.NoResize;
+        if (!showTitleBar)
+            flags |= ImGuiWindowFlags.NoTitleBar;
         return flags;
     }
 
@@ -114,7 +116,11 @@ internal sealed class MainWindow : Window
                 BgAlpha = null;
         }
 
-        Flags = ResolveFlags(Plugin.Config.CanMove, Plugin.Config.CanResize);
+        Flags = ResolveFlags(
+            Plugin.Config.CanMove,
+            Plugin.Config.CanResize,
+            Plugin.Config.ShowTitleBar
+        );
     }
 
     public Tab? ActiveTab => _activeTab;