fix: clamp negative resources + add tutorial focus trap

- Clamp ops, trust, compute to >= 0 in tick() to prevent negative
  values from Fenrir drain and debuff effects
- Add keyboard focus trap to tutorial overlay for accessibility
  (prevents Tab from escaping the dialog)
- Clean up focus trap handler on tutorial close

js/tutorial.js

@@ -208,6 +208,23 @@ function renderTutorialStep(index) {
     // Focus the next button so Enter works
     const nextBtn = document.getElementById('tutorial-next-btn');
     if (nextBtn) nextBtn.focus();
+
+    // Focus trap: prevent tabbing outside the tutorial overlay
+    overlay._focusTrapHandler = function(e) {
+        if (e.key !== 'Tab') return;
+        const focusable = overlay.querySelectorAll('button, [href], [tabindex]:not([tabindex="-1"])');
+        if (focusable.length === 0) return;
+        const first = focusable[0];
+        const last = focusable[focusable.length - 1];
+        if (e.shiftKey && document.activeElement === first) {
+            e.preventDefault();
+            last.focus();
+        } else if (!e.shiftKey && document.activeElement === last) {
+            e.preventDefault();
+            first.focus();
+        }
+    };
+    overlay.addEventListener('keydown', overlay._focusTrapHandler);
 }
 
 let _tutorialStep = 0;
@@ -236,6 +253,9 @@ document.addEventListener('keydown', function tutorialKeyHandler(e) {
 function closeTutorial() {
     const overlay = document.getElementById('tutorial-overlay');
     if (overlay) {
+        if (overlay._focusTrapHandler) {
+            overlay.removeEventListener('keydown', overlay._focusTrapHandler);
+        }
         overlay.style.animation = 'tutorial-fade-in 0.3s ease-in reverse';
         setTimeout(() => overlay.remove(), 280);
     }