refactor(fonts): hybrid FontManager init via SuppressAutoRebuild

Move font handle creation from BuildFonts() into the FontManager ctor
inside a single atlas.SuppressAutoRebuild() block. Axis, AxisItalic and
FontAwesome become init-only IFontHandle properties; RegularFont and
ItalicFont stay mutable so the live font-settings rebuild path keeps
working without a plugin reload.

- BuildFonts() renamed to RebuildDelegateFonts(), scope reduced to the
  delegate fonts only
- BuildFontsAsync() removed; Task.Run had no purpose with ctor-init
- FontManagerInitHostedService deleted; PluginHostFactory drops the
  matching AddHostedService registration
- PluginHostFactory FontManager registration takes IDalamudPluginInterface
  via factory lambda
- Settings save path now calls RebuildDelegateFonts() instead of
  BuildFonts()
- Plugin.Draw push site gets a null-forgiving for the nullable
  RegularFont with a one-line WHY

HellionChat/Ui/Settings.cs

@@ -229,7 +229,7 @@ public sealed class SettingsWindow : Dalamud.Interface.Windowing.Window
         }
 
         if (fontChanged || fontSizeChanged || italicStateChanged)
-            Plugin.FontManager.BuildFonts();
+            Plugin.FontManager.RebuildDelegateFonts();
 
         if (languageChanged)
             Plugin.LanguageChanged(Plugin.Interface.UiLanguage);