Add utilized volume level indicator and fix water fill level

Water fill height now reflects water_mass + grain_mass*0.7 (grain
displacement). A small blue tick and label on the left pot wall shows
the utilized volume in litres, hidden when the pot is unconfigured.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JLR2133M2PyjiYq584i2G7

web/index.html

@@ -144,6 +144,12 @@
     <line x1="60"  y1="174" x2="180" y2="174" stroke="#bbb" stroke-width="4" stroke-linecap="round"/>
     <line x1="60"  y1="28"  x2="180" y2="28"  stroke="#bbb" stroke-width="4" stroke-linecap="round"/>
 
+    <!-- Utilized volume level indicator (JS-positioned) -->
+    <line id="level-tick" x1="52" y1="172" x2="60" y2="172"
+          stroke="#4a9ed8" stroke-width="1.5" stroke-linecap="round" style="display:none"/>
+    <text id="level-label" x="50" y="172" font-size="8" fill="#4a9ed8"
+          dominant-baseline="middle" text-anchor="end" style="display:none">0.0 L</text>
+
     <!-- Stirrer blades (inside pot) -->
     <g id="stirrer-blades" clip-path="url(#pot-clip)">
       <rect x="87"  y="133" width="31" height="5" fill="#666" rx="1"/>