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/start_menu.gd

@@ -11,6 +11,7 @@ signal settings_requested()
 signal quit_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")
 
 const VENUES := [
@@ -108,7 +109,7 @@ func _build() -> void:
 	_continue_button.name = "ContinueButton"
 	_continue_button.custom_minimum_size = Vector2(460, 52)
 	_continue_button.pressed.connect(_on_continue_pressed)
-	_style_venue_button(_continue_button)
+	UiStyles.apply_button(_continue_button, 18)
 	column.add_child(_continue_button)
 
 	for i in VENUES.size():
@@ -119,7 +120,7 @@ func _build() -> void:
 		button.alignment = HORIZONTAL_ALIGNMENT_LEFT
 		button.custom_minimum_size = Vector2(460, 62)
 		button.pressed.connect(_on_venue_pressed.bind(i))
-		_style_venue_button(button)
+		UiStyles.apply_button(button, 18)
 		column.add_child(button)
 		_buttons.append(button)
 
@@ -136,18 +137,21 @@ func _build() -> void:
 	help_button.name = "HelpButton"
 	help_button.text = Strings.lookup("ui.start_help")
 	help_button.pressed.connect(func(): help_requested.emit())
+	UiStyles.apply_button(help_button, 16)
 	row.add_child(help_button)
 
 	var settings_button := Button.new()
 	settings_button.name = "SettingsButton"
 	settings_button.text = Strings.lookup("ui.start_settings")
 	settings_button.pressed.connect(func(): settings_requested.emit())
+	UiStyles.apply_button(settings_button, 16)
 	row.add_child(settings_button)
 
 	var quit_button := Button.new()
 	quit_button.name = "QuitButton"
 	quit_button.text = Strings.lookup("ui.quit_game")
 	quit_button.pressed.connect(func(): quit_requested.emit())
+	UiStyles.apply_button(quit_button, 16)
 	row.add_child(quit_button)
 
 	var hint := Label.new()
@@ -158,35 +162,6 @@ func _build() -> void:
 
 	_focus_index(0)
 
-## Match the dark, rounded rows in the `start_menu` mockup.
-func _style_venue_button(button: Button) -> void:
-	button.add_theme_font_size_override("font_size", 18)
-	button.add_theme_color_override("font_color", Color(0.94, 0.94, 0.92))
-	button.add_theme_color_override("font_hover_color", Color.WHITE)
-	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
-	normal.content_margin_left = 22
-	normal.content_margin_right = 22
-	normal.content_margin_top = 10
-	normal.content_margin_bottom = 10
-	var hover := normal.duplicate() as StyleBoxFlat
-	hover.bg_color = Color(0.16, 0.175, 0.21, 0.98)
-	var focus := normal.duplicate() as StyleBoxFlat
-	focus.bg_color = Color(0.14, 0.155, 0.19, 0.98)
-	focus.border_color = Color(0.85, 0.86, 0.88)
-	focus.border_width_left = 2
-	focus.border_width_top = 2
-	focus.border_width_right = 2
-	focus.border_width_bottom = 2
-	button.add_theme_stylebox_override("normal", normal)
-	button.add_theme_stylebox_override("hover", hover)
-	button.add_theme_stylebox_override("focus", focus)
-	button.add_theme_stylebox_override("pressed", hover)
-
 func _focus_index(i: int) -> void:
 	if _buttons.is_empty():
 		return