feat(web): add game settings screen and submit confirmation

- Add GameSetup component with Duolingo-style button selectors for
  language pair, POS, difficulty, and rounds
- Language swap: selecting the same language for source and target
  automatically swaps them instead of allowing duplicates
- Add selection-before-submission flow: user clicks an option to
  highlight it, then confirms with a Submit button to prevent misclicks
- Add selected state to OptionButton (purple ring highlight)
- Play Again on score screen returns to settings instead of
  auto-restarting with the same configuration
- Remove hardcoded GAME_SETTINGS, game configuration is now user-driven

apps/web/src/components/game/OptionButton.tsx

@@ -1,6 +1,6 @@
 type OptionButtonProps = {
   text: string;
-  state: "idle" | "disabled" | "correct" | "wrong";
+  state: "idle" | "selected" | "disabled" | "correct" | "wrong";
   onSelect: () => void;
 };
 
@@ -10,6 +10,8 @@ export const OptionButton = ({ text, state, onSelect }: OptionButtonProps) => {
 
   const styles = {
     idle: "bg-white text-purple-900 border-purple-200 hover:bg-purple-50 hover:border-purple-300 hover:-translate-y-0.5 active:translate-y-0 active:border-b-2",
+    selected:
+      "bg-purple-100 text-purple-900 border-purple-400 ring-2 ring-purple-400",
     disabled: "bg-gray-100 text-gray-400 border-gray-200 cursor-default",
     correct: "bg-emerald-400 text-white border-emerald-600 scale-[1.02]",
     wrong: "bg-pink-400 text-white border-pink-600",
@@ -19,7 +21,9 @@ export const OptionButton = ({ text, state, onSelect }: OptionButtonProps) => {
     <button
       className={`${base} ${styles[state]}`}
       onClick={onSelect}
-      disabled={state !== "idle"}
+      disabled={
+        state === "disabled" || state === "correct" || state === "wrong"
+      }
     >
       {text}
     </button>