fix: compilation errors with cpp_std=c++20

There were two main issues with fmtlib and C++20 mode:

 - `fmt::format` defaults to compile-time argument checking and requires
   using `fmt::runtime(format_string)` to bypass that.
 - `std::format` implementation introduces conflicting declarations and
   we have to specify the namespace for all `format`/`format_to` calls.

src/modules/river/mode.cpp

@@ -103,7 +103,7 @@ void Mode::handle_mode(const char *mode) {
     }
 
     label_.get_style_context()->add_class(mode);
-    label_.set_markup(fmt::format(format_, Glib::Markup::escape_text(mode).raw()));
+    label_.set_markup(fmt::format(fmt::runtime(format_), Glib::Markup::escape_text(mode).raw()));
     label_.show();
   }