- Replaced the dark mode checkbox with an Auto/Dark/Light selector in Preferences → Colors and added the new mode labels/setting metadata to match the combo box UI.

- Updated palette handling to respect the effective dark mode (including auto) when saving colors, applying themes, and refreshing user list styling.
- Added auto dark-mode tracking that listens to system theme changes and reapplies palette/styles live when Auto is selected, so updates happen without restart (including channel list styling updates via setup_apply_real).
- Synced the stored auto dark-mode state when preferences are applied, keeping Auto mode consistent after manual changes.
- Exposed a helper for keeping the Auto state synchronized from the GTK layer to preferences handling.

src/fe-gtk/fe-gtk.h

@@ -184,6 +184,7 @@ extern cairo_surface_t *dialogwin_pix;
 
 gboolean fe_dark_mode_is_enabled (void);
 gboolean fe_dark_mode_is_enabled_for (unsigned int mode);
+void fe_set_auto_dark_mode_state (gboolean enabled);
 
 #define SPELL_ENTRY_GET_TEXT(e) ((char *)(gtk_entry_get_text (GTK_ENTRY(e))))
 #define SPELL_ENTRY_SET_TEXT(e,txt) gtk_entry_set_text(GTK_ENTRY(e),txt)