Unify action-button states across the UI

- Add UiStyles with explicit normal, hover, pressed, focus and disabled styles and apply it to every action button.
- Disable timeline Previous/Next at the first/last event and keep Close always available.
- Style settings reset-dialog buttons and cover the button-state audit in docs/ui-copy.md.
- Add shared-state and timeline-boundary regression tests.

src/game/ui/radio_panel.gd

@@ -10,6 +10,7 @@ signal cancelled()
 const Rules := preload("res://src/core/rules.gd")
 const Strings := preload("res://src/core/strings.gd")
 const UiArt := preload("res://src/game/ui/art.gd")
+const UiStyles := preload("res://src/game/ui/ui_styles.gd")
 
 var panel: PanelContainer
 var npc_name_label: Label
@@ -107,10 +108,12 @@ func _init() -> void:
 	action_button.button_down.connect(func(): _start_hold())
 	action_button.button_up.connect(func(): _stop_hold(false))
 	action_button.pressed.connect(_on_action_pressed)
+	UiStyles.apply_button(action_button, 16)
 	buttons.add_child(action_button)
 	var cancel_btn := Button.new()
 	cancel_btn.text = Strings.lookup("ui.radio_cancel")
 	cancel_btn.pressed.connect(func(): cancelled.emit())
+	UiStyles.apply_button(cancel_btn, 16)
 	buttons.add_child(cancel_btn)
 
 var _title_label: Label