Add animated fire visualization and fix step pot param reading

- SVG fire group with 3 layered flame paths (red/orange/yellow gradients)
- CSS keyframe flicker animations with different timings per flame layer
- JS controls fire opacity and scaleY proportional to heater power
- Fix buildStep() reading grain_mass/water_mass from nested step.pot
  instead of flat step keys, matching the JSON structure

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

web/style.css

@@ -152,6 +152,39 @@ header#connection-bar {
     display: block;
 }
 
+/* Fire animations */
+@keyframes flicker-1 {
+  0%, 100% { transform: scaleY(1.00) scaleX(1.00); }
+  25%       { transform: scaleY(1.09) scaleX(0.93); }
+  50%       { transform: scaleY(0.93) scaleX(1.06); }
+  75%       { transform: scaleY(1.06) scaleX(0.96); }
+}
+@keyframes flicker-2 {
+  0%, 100% { transform: scaleY(0.95) scaleX(1.04); }
+  30%       { transform: scaleY(1.10) scaleX(0.92); }
+  65%       { transform: scaleY(0.91) scaleX(1.07); }
+}
+@keyframes flicker-3 {
+  0%, 100% { transform: scaleY(1.05) scaleX(0.95); }
+  40%       { transform: scaleY(0.89) scaleX(1.08); }
+  70%       { transform: scaleY(1.12) scaleX(0.91); }
+}
+
+.flame {
+    transform-box: fill-box;
+    transform-origin: 50% 100%;
+}
+#flame-outer { animation: flicker-1 0.70s ease-in-out infinite; }
+#flame-mid   { animation: flicker-2 0.55s ease-in-out infinite 0.08s; }
+#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;
+}
+
 /* --- Tabs --- */
 
 nav#tabs {