Add window opacity slider to the Hellion theme

True per-window focus-aware transparency would require touching
ChatLogWindow and SettingsWindow individually (both upstream code,
both prone to cherry-pick churn). Instead expose a single opacity
slider that mixes a configured alpha into the WindowBg color in
PushGlobal — applies to every Hellion-rendered pane uniformly,
the game shines through, and form fields / dialogs / popups stay
opaque on top so input remains readable.

Default 92%, range clamped to 0.5–1.0 in the UI and 0x55–0xFF in
the alpha conversion so users can't accidentally make the panes
disappear entirely. Slider sits inside the Appearance section
right under the master Hellion-theme checkbox and is greyed out
when the theme is disabled.

ChatTwo/Ui/SettingsTabs/Privacy.cs

@@ -77,6 +77,15 @@ internal sealed class Privacy : ISettingsTab
                 ref Mutable.HellionThemeEnabled,
                 HellionStrings.Theme_Enabled_Name,
                 HellionStrings.Theme_Enabled_Description);
+
+            using (ImRaii.Disabled(!Mutable.HellionThemeEnabled))
+            {
+                ImGui.Spacing();
+                var opacity = Mutable.HellionThemeWindowOpacity;
+                if (ImGui.SliderFloat($"{HellionStrings.Theme_WindowOpacity_Label}##theme-opacity", ref opacity, 0.5f, 1.0f, "%.2f"))
+                    Mutable.HellionThemeWindowOpacity = Math.Clamp(opacity, 0.5f, 1.0f);
+                ImGuiUtil.HelpText(HellionStrings.Theme_WindowOpacity_Help);
+            }
         }
 
         ImGui.Spacing();