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/test/tests/test_ui_buttons.gd

@@ -0,0 +1,26 @@
+extends RefCounted
+## Shared UI button states: every normal/hover/pressed/focus/disabled state is
+## explicitly styled so the custom dark panels do not fall back to the engine
+## theme when a control is focused or disabled.
+
+const UiStyles := preload("res://src/game/ui/ui_styles.gd")
+
+func test_shared_button_states_are_complete() -> bool:
+	var button := Button.new()
+	UiStyles.apply_button(button, 16)
+	var states := ["normal", "hover", "pressed", "focus", "disabled"]
+	var missing: Array[String] = []
+	for state in states:
+		if not button.has_theme_stylebox_override(state):
+			missing.append(state)
+	var a := TestUtils.check(missing.is_empty(),
+		"all five button states have explicit styles (missing: %s)" % str(missing))
+	var b := TestUtils.check_eq(button.get_theme_font_size("font_size"), 16,
+		"the shared style applies the requested font size")
+	var disabled := button.get_theme_stylebox("disabled")
+	var normal := button.get_theme_stylebox("normal")
+	var c := TestUtils.check(disabled != null and normal != null
+		and disabled.bg_color != normal.bg_color,
+		"the disabled state is visually distinct from normal")
+	button.free()
+	return a and b and c