Add pot/thermometer visualization to browser GUI

- Add volumen field to all sud JSON pot entries (30 L)
- Fix parseSudDoc to read from nested doc.pot (was using stale flat keys)
- SVG widget shows transparent rectangular pot with stirrer, thermometer
  with live temperature, and water fill level (water_mass/volumen) when
  a sud is loaded; no water shown when no sud is loaded

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

web/style.css

@@ -133,6 +133,18 @@ header#connection-bar {
     border-radius: 3px;
 }
 
+/* --- Pot visualization --- */
+
+#pot-view {
+    margin: 0.5em 0 0.5em 0;
+}
+#pot-svg {
+    width: 100%;
+    max-width: 180px;
+    height: auto;
+    display: block;
+}
+
 /* --- Tabs --- */
 
 nav#tabs {