Add win condition UI and zero-G grid level

ui.js

@@ -14,11 +14,18 @@
     const restartBtn = document.getElementById("restart-btn");
     const pauseBtn = document.getElementById("pause-btn");
     const pauseOverlay = document.getElementById("pause-overlay");
+    const goalLabelEl = document.getElementById("goal-label");
+    const goalProgressEl = document.getElementById("goal-progress");
+    const winEl = document.getElementById("win-overlay");
+    const winMessageEl = document.getElementById("win-message");
+    const winNextBtn = document.getElementById("win-next");
+    const winRestartBtn = document.getElementById("win-restart");
 
     const handlers = {
       onPauseToggle: null,
       onRestart: null,
       onSceneChange: null,
+      onWinNext: null,
     };
 
     if (pauseBtn) {
@@ -39,6 +46,18 @@
       });
     }
 
+    if (winNextBtn) {
+      winNextBtn.addEventListener("click", () => {
+        if (handlers.onWinNext) handlers.onWinNext();
+      });
+    }
+
+    if (winRestartBtn) {
+      winRestartBtn.addEventListener("click", () => {
+        if (handlers.onRestart) handlers.onRestart();
+      });
+    }
+
     window.addEventListener("keydown", (e) => {
       if (e.key === "Escape" && handlers.onPauseToggle) {
         handlers.onPauseToggle();
@@ -78,6 +97,24 @@
       }
     };
 
+    const setGoal = ({ label, progress }) => {
+      if (goalLabelEl) goalLabelEl.textContent = label || "—";
+      if (goalProgressEl)
+        goalProgressEl.style.width = `${Math.max(
+          0,
+          Math.min(100, progress ?? 0),
+        )}%`;
+    };
+
+    const showWin = (message) => {
+      if (winMessageEl) winMessageEl.textContent = message || "You win!";
+      if (winEl) winEl.classList.add("visible");
+    };
+
+    const hideWin = () => {
+      if (winEl) winEl.classList.remove("visible");
+    };
+
     const showGameOver = (score) => {
       if (finalScoreEl) finalScoreEl.textContent = score;
       if (gameOverEl) gameOverEl.classList.add("visible");
@@ -146,9 +183,12 @@
       setPauseState,
       showGameOver,
       hideGameOver,
+      showWin,
+      hideWin,
       setSceneOptions,
       setSceneSelection,
       setHandlers,
+      setGoal,
     };
   };