[Fonts] Make the editor's fallback and Reset preview-independent

- Add ThemeManager::defaultAppFont(), which resolves an unset role against the
  startup font instead of currentFonts, and use it in FontGridWidget::loadFonts
  so the grid never shows the live preview as the theme's default.
- Reload the live theme in PaletteEditorDialog::onReset before re-reading the
  grid, so Reset restores the saved fonts before loadFonts resolves fallbacks.

cockatrice/src/interface/theme_manager.cpp

@@ -663,6 +663,14 @@ QFont ThemeManager::appFont(AppFont::Role role) const
     return qApp->font();
 }
 
+QFont ThemeManager::defaultAppFont(AppFont::Role /*role*/) const
+{
+    // No per-role scaling and no preview: an unset role resolves to the font
+    // captured at startup, which is what the theme falls back to once the
+    // editor closes.
+    return defaultFont;
+}
+
 void ThemeManager::themeChangedSlot()
 {
     QString themeName = SettingsCache::instance().getThemeName();