[Fonts] Keep the DeckList fallback on the application font

DeckList is read by the deck editor headers, the filter tree and the deck
view too, not only by the two surfaces that hardcoded Serif. Resolving the
unset role to Serif in appFont() made every user without a fonts.toml see
Serif outside the deck list.

- appFont(DeckList) falls back to the application font again.
- ThemeManager::deckListFont() resolves the role and keeps the Serif
  fallback for the two historical sites: the deck view zone headers and
  the printed deck list.
- DeckView no longer repaints its viewport on fontsChanged; touching the
  scene items already schedules the repaint in every attached view.

cockatrice/src/interface/theme_manager.h

@@ -150,15 +150,20 @@ public:
     // when present, otherwise a palette-accent-derived fallback.
     QColor appColor(AppColor::Role role) const;
     /** @brief Resolves an application font role: the theme's stored [Fonts]
-     *         value when present, otherwise a per-role fallback (monospace for
-     *         ChatLog, serif basis for CardText and DeckList, the application
-     *         font for the remaining roles). */
+     *         value when present, otherwise the application font. Roles that
+     *         had a historical hardcoded family ask for it through
+     *         cardTextFont() and deckListFont() instead. */
     QFont appFont(AppFont::Role role) const;
     /** @brief Whether the active theme defines this role in fonts.toml. */
     bool hasAppFont(AppFont::Role role) const;
     /** @brief The CardText role with its historical Serif fallback. Used by the
      *         counter painters, which were Serif before the role existed. */
     QFont cardTextFont() const;
+    /** @brief The DeckList role with its historical Serif fallback. Used by the
+     *         two surfaces that hardcoded Serif before the role existed (the
+     *         deck view zone headers and the printed deck list); every other
+     *         DeckList reader falls back to the application font. */
+    QFont deckListFont() const;
     /** @brief Resolves an application font role against the startup font only,
      *         ignoring any previewed config in currentFonts. Editors that show
      *         "what this role resolves to" must use this, because appFont()