burn: fix null ref in renderResources and add tutorial dialog a11y

BUG-08: Add null check on closest('.res') in renderResources to
prevent TypeError if DOM structure is unexpected.

BUG-11: Add role='dialog', aria-modal='true', aria-label='Tutorial'
to tutorial overlay. Add aria-label to Skip and Next buttons for
screen reader accessibility.

Smoke test: all 19 checks passed.

js/tutorial.js

@@ -177,6 +177,9 @@ function renderTutorialStep(index) {
     if (!overlay) {
         overlay = document.createElement('div');
         overlay.id = 'tutorial-overlay';
+        overlay.setAttribute('role', 'dialog');
+        overlay.setAttribute('aria-modal', 'true');
+        overlay.setAttribute('aria-label', 'Tutorial');
         document.body.appendChild(overlay);
     }
 
@@ -196,8 +199,8 @@ function renderTutorialStep(index) {
             <div class="t-tip">${step.tip}</div>
             <div id="tutorial-dots">${dots}</div>
             <div id="tutorial-btns">
-                <button id="tutorial-skip-btn" onclick="closeTutorial()">Skip</button>
-                <button id="tutorial-next-btn" onclick="${isLast ? 'closeTutorial()' : 'nextTutorialStep()'}">${isLast ? 'Start Playing' : 'Next →'}</button>
+                <button id="tutorial-skip-btn" onclick="closeTutorial()" aria-label="Skip tutorial">Skip</button>
+                <button id="tutorial-next-btn" onclick="${isLast ? 'closeTutorial()' : 'nextTutorialStep()'}" aria-label="${isLast ? 'Start playing' : 'Next tutorial step'}">${isLast ? 'Start Playing' : 'Next →'}</button>
             </div>
         </div>
     `;