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/event_timeline.gd

@@ -9,12 +9,15 @@ signal closed()
 
 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 events: Array[Dictionary] = []
 var index := 0
 var _list_label: Label = null
 var _detail_label: Label = null
 var _counter_label: Label = null
+var _prev_button: Button = null
+var _next_button: Button = null
 var _root: Control = null
 
 func _init() -> void:
@@ -90,22 +93,25 @@ func _build() -> void:
 	row.add_theme_constant_override("separation", 10)
 	column.add_child(row)
 
-	var prev := Button.new()
-	prev.name = "PrevButton"
-	prev.text = Strings.lookup("timeline.prev")
-	prev.pressed.connect(func(): step(-1))
-	row.add_child(prev)
+	_prev_button = Button.new()
+	_prev_button.name = "PrevButton"
+	_prev_button.text = Strings.lookup("timeline.prev")
+	_prev_button.pressed.connect(func(): step(-1))
+	UiStyles.apply_button(_prev_button, 15, true)
+	row.add_child(_prev_button)
 
-	var next := Button.new()
-	next.name = "NextButton"
-	next.text = Strings.lookup("timeline.next")
-	next.pressed.connect(func(): step(1))
-	row.add_child(next)
+	_next_button = Button.new()
+	_next_button.name = "NextButton"
+	_next_button.text = Strings.lookup("timeline.next")
+	_next_button.pressed.connect(func(): step(1))
+	UiStyles.apply_button(_next_button, 15, true)
+	row.add_child(_next_button)
 
 	var close := Button.new()
 	close.name = "CloseButton"
 	close.text = Strings.lookup("timeline.close")
 	close.pressed.connect(_on_close)
+	UiStyles.apply_button(close, 15, true)
 	row.add_child(close)
 
 	var hint := Label.new()
@@ -154,7 +160,12 @@ func _on_close() -> void:
 	close()
 
 func _refresh() -> void:
-	if events.is_empty():
+	var has_events := not events.is_empty()
+	if _prev_button:
+		_prev_button.disabled = not has_events or index <= 0
+	if _next_button:
+		_next_button.disabled = not has_events or index >= events.size() - 1
+	if not has_events:
 		_counter_label.text = Strings.lookup("timeline.empty")
 		_detail_label.text = ""
 		_list_label.text = ""