feat(keyboard-layout): custom labels support (v5 port)

[fennstef]

Summary

Port of the keyboard layout custom labels feature from v4 to v5.

• DisplayMode::Custom: New display mode that shows a user-defined label for each layout. Falls back to the layout name if no mapping is configured.
• custom_labels config key: TOML sub-table under [widget.keyboard_layout] mapping layout names to display labels:

  [widget.keyboard_layout]
  display = "custom"
  [widget.keyboard_layout.custom_labels]
  de = "DE"
  us = "EN"

• Settings GUI: All keyboard layout settings are now exposed in the widget inspector (cycle_command, hide_when_single_layout, show_icon, show_label, display, custom_labels). The custom_labels editor pre-populates rows for all currently active compositor layouts.
• OSD pill: The layout-change notification pill also uses custom labels.

Implementation notes

custom_labels is a string→string map. Because toml++ statically introspects the WidgetSettingValue variant and rejects non-native TOML types, the map cannot be added to that variant. Instead, a separate tables field (unordered_map<string, unordered_map<string,string>>) was added to WidgetConfig to hold TOML sub-tables. Override persistence works via widgetConfigEqual which now compares tables as well.

Test plan

• Set display = "custom" in widget inspector, verify segmented control shows short/full/custom
• Active layout names appear pre-populated in the custom labels editor
• Enter a label, press Enter → verify saved in ~/.local/state/noctalia/settings.toml
• Close and reopen settings → labels still present
• Switch keyboard layout → OSD pill shows custom label
• With no custom label set for a layout → OSD falls back to display mode formatting