Add soll temperature marker to thermometer

Orange tick and label on the left side of the tube, dynamically
positioned via plotTempSoll updated from TempCtrl Soll messages.

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

web/index.html

@@ -170,6 +170,12 @@
     <rect id="thermo-mercury" x="215" y="174" width="8" height="0"
           fill="#e0190a" clip-path="url(#tube-clip)"/>
 
+    <!-- Soll temperature marker (left side, orange, JS-positioned) -->
+    <line id="thermo-soll-tick" x1="203" y1="174" x2="212" y2="174"
+          stroke="#ff7043" stroke-width="2" stroke-linecap="round"/>
+    <text id="thermo-soll-label" x="201" y="174" font-size="9"
+          fill="#ff7043" text-anchor="end" dominant-baseline="middle"></text>
+
     <!-- Scale ticks -->
     <line x1="227" y1="28"  x2="236" y2="28"  stroke="#9e9e9e" stroke-width="1.5"/>
     <text x="239" y="32"  font-size="9" fill="#9e9e9e">100</text>