Fix empty layout display for custom XKB layouts in sway/language module

[nick133]

Problem

Custom XKB layouts (e.g. defined in ~/.config/xkb/symbols/) are not present in libxkbregistry.
When such a layout is active, sway/language module fails to resolve it in layouts_map_, resulting in empty output for all layout placeholders ({short}, {long}, {shortDescription}).

Cause

The code previously used layouts_map_[current_layout], which assumes every active layout exists in layouts_map_:

layout_ = layouts_map_[current_layout];

For unknown layouts, this produces a default Layout with empty fields.

Fix

Add a fallback for unknown layouts instead of relying on map::operator[].

static std::string fallback_layout_short_name(std::string_view full_name) {
  const size_t n = std::min<size_t>(2, full_name.size());
  return std::string(full_name.substr(0, n));
}

auto Language::set_current_layout(const std::string& current_layout) -> void {
  label_.get_style_context()->remove_class(layout_.short_name);

  auto it = layouts_map_.find(current_layout);

  if (it != layouts_map_.end()) {
    layout_ = it->second;
  } else {
    spdlog::debug("Language: using fallback for unknown layout '{}'", current_layout);

    auto short_name = fallback_layout_short_name(current_layout);

    layout_ = Layout{
      current_layout,
      short_name,
      "",
      short_name
    };
  }

  label_.get_style_context()->add_class(layout_.short_name);
}

Notes

• No change for layouts found in libxkbregistry
• Fixes empty output for all layout format fields
• Improves handling of custom XKB configurations