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/pause_overlay.gd

@@ -10,6 +10,7 @@ signal settings_requested()
 signal quit_to_menu_requested()
 
 const UiArt := preload("res://src/game/ui/art.gd")
+const UiStyles := preload("res://src/game/ui/ui_styles.gd")
 const Strings := preload("res://src/core/strings.gd")
 
 var _root: Control = null
@@ -67,19 +68,7 @@ func _add_button(parent: Control, text: String, node_name: String, cb: Callable)
 	button.name = node_name
 	button.text = text
 	button.custom_minimum_size = Vector2(380, 52)
-	button.add_theme_font_size_override("font_size", 22)
-	var normal := StyleBoxFlat.new()
-	normal.bg_color = Color(0.115, 0.125, 0.15, 0.96)
-	normal.corner_radius_top_left = 12
-	normal.corner_radius_top_right = 12
-	normal.corner_radius_bottom_left = 12
-	normal.corner_radius_bottom_right = 12
-	var hover := normal.duplicate() as StyleBoxFlat
-	hover.bg_color = Color(0.16, 0.175, 0.21, 0.98)
-	button.add_theme_stylebox_override("normal", normal)
-	button.add_theme_stylebox_override("hover", hover)
-	button.add_theme_stylebox_override("focus", hover)
-	button.add_theme_stylebox_override("pressed", hover)
+	UiStyles.apply_button(button, 22)
 	button.pressed.connect(cb)
 	parent.add_child(button)
 	return button