burn: add missing phase-transition overlay element

Fixes #101 (Bug 2 of 3).

showPhaseTransition() expected a #phase-transition element with
children .pt-phase, .pt-name, .pt-desc. The element was missing
from index.html, causing phase transitions to silently fail.

Added the overlay div and matching CSS .active state, styled
after the existing #drift-ending pattern.

Note: Bugs 1 (Memory Leak toast) and 3 (mute/contrast buttons)
were already fixed in PR #103.

index.html

@@ -79,6 +79,8 @@ body{background:var(--bg);color:var(--text);font-family:'SF Mono','Cascadia Code
 .save-btn,.reset-btn{font-size:10px;text-align:center;padding:6px 10px;border-radius:4px;cursor:pointer;font-family:inherit;width:100%;margin-top:4px;transition:all 0.15s}
 .save-btn{background:var(--border);border:1px solid #252540;color:var(--text)}.save-btn:hover{border-color:var(--accent)}
 .reset-btn{background:#1a0808;border:1px solid #3a1a1a;color:var(--red)}.reset-btn:hover{background:#2a1010}
+#phase-transition{display:none}
+#phase-transition.active{display:flex}
 #drift-ending{display:none;position:fixed;top:0;left:0;right:0;bottom:0;background:rgba(8,8,16,0.97);z-index:100;justify-content:center;align-items:center;flex-direction:column;text-align:center;padding:40px}
 #drift-ending.active{display:flex}
 #drift-ending h2{font-size:20px;color:#f44336;letter-spacing:4px;margin-bottom:20px;font-weight:300}
@@ -272,6 +274,11 @@ The light is on. The room is empty."
 </div>
 </div>
 
+<div id="phase-transition" style="display:none;position:fixed;top:0;left:0;right:0;bottom:0;background:rgba(8,8,16,0.95);z-index:95;justify-content:center;align-items:center;flex-direction:column;text-align:center;padding:40px;pointer-events:none">
+  <div class="pt-phase" style="font-size:12px;color:var(--dim);letter-spacing:4px;margin-bottom:12px">PHASE</div>
+  <div class="pt-name" style="font-size:28px;font-weight:300;color:var(--gold);letter-spacing:4px;text-shadow:0 0 40px #ffd70044;margin-bottom:8px"></div>
+  <div class="pt-desc" style="font-size:12px;color:var(--dim);font-style:italic;max-width:400px"></div>
+</div>
 <div id="toast-container"></div>
 <div id="custom-tooltip"></div>
 </body>