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.

content/strings.en.json

@@ -108,5 +108,7 @@
   "ui.radio_cancelled": "Call cancelled — hold the button.",
   "ui.radio_holding": "Holding… %.0f%%",
   "ui.review_return_menu": "Return to menu",
+  "ui.quit_game": "Quit game",
+  "ui.apply_settings": "Apply settings",
   "tutorial.skipped": "Tutorial skipped."
 }