refactor: migrate complex-tooltip modules to ALabel dedup setters

Switch direct label_.set_markup/set_tooltip_markup calls to the
deduplicating setLabelMarkup/setTooltipMarkup helpers in clock, custom
and mpris, keeping their custom tooltip construction logic intact.
taskbar is left untouched: its tooltip lives on per-task Gtk::Button,
not on an ALabel label_.

src/modules/mpris/mpris.cpp

@@ -735,7 +735,7 @@ auto Mpris::update() -> void {
     if (label_format.empty()) {
       label_.hide();
     } else {
-      label_.set_markup(label_format);
+      setLabelMarkup(label_format);
       label_.show();
     }
   } catch (fmt::format_error const& e) {
@@ -758,7 +758,7 @@ auto Mpris::update() -> void {
           fmt::arg("player_icon", getIconFromJson(config_["player-icons"], info.name)),
           fmt::arg("status_icon", getIconFromJson(config_["status-icons"], info.status_string)));
 
-      label_.set_tooltip_markup(tooltip_text);
+      setTooltipMarkup(tooltip_text);
     } catch (fmt::format_error const& e) {
       spdlog::warn("mpris: format error (tooltip): {}", e.what());
     }