feat: rework percentile roll into a stateful tens+ones flow

Tens and Ones now combine into a real 1-100 percentile result
(00+0 = 100) instead of rolling independently. Ones is disabled
until Tens is rolled, and a pending visual state shows the tens
value while waiting for the ones roll. Adds a Clear button to
reset the flow.

public/index.html

@@ -273,10 +273,11 @@
           <div class="card-title">Percentile</div>
           <div class="dice-actions-row">
             <button class="roll-btn" id="percentileTensBtn">Tens</button>
-            <button class="roll-btn" id="percentileOnesBtn">Ones</button>
+            <button class="roll-btn" id="percentileOnesBtn" disabled>Ones</button>
+            <button class="clear-btn" id="percentileClearBtn">Clear</button>
           </div>
           <div class="dice-result-box" style="margin-top:1rem;" id="percentileResult">
-            <span class="placeholder">Roll tens or ones.</span>
+            <span class="placeholder">Roll tens to begin.</span>
           </div>
         </div>