Standardize button labels and document UI states

Finish the button-label audit with a surface-by-surface table, clarify Quit
game vs Quit to menu, rename Apply to Apply settings, and use string-table
keys for the new labels. Extend the smoke checklist with UI copy/button checks.
Headless 250/250, docs check OK.

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 Strings := preload("res://src/core/strings.gd")
 
 const VENUES := [
 	{"id": "hollywood_building", "label": "Hollywood Boulevard", "blurb": "Building entrance, tourists and late-night trouble."},
@@ -142,7 +143,7 @@ func _build() -> void:
 
 	var quit_button := Button.new()
 	quit_button.name = "QuitButton"
-	quit_button.text = "Quit"
+	quit_button.text = Strings.lookup("ui.quit_game")
 	quit_button.pressed.connect(func(): quit_requested.emit())
 	row.add_child(quit_button)