Fix fire scaling: clip upper flames, keep base constant

Replace scaleY group transform with a dynamic clipPath that reveals
flame height proportional to heater power. Base at y=256 stays fixed;
only the tips grow with intensity. Includes flicker overshoot buffer.
Fire paths shifted +8px down, viewBox extended to 265.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DGKe8JzTM2Gr1fdx9wYXTi

web/style.css

@@ -206,8 +206,6 @@ header#connection-bar {
 #flame-inner { animation: flicker-3 0.45s ease-in-out infinite 0.15s; }
 
 #fire-group {
-    transform-box: fill-box;
-    transform-origin: 50% 100%;
     opacity: 0;
     transition: opacity 0.4s;
 }