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

sude/GuiTest.json

@@ -7,7 +7,8 @@
 		"L": 0.2,
 		"Td": 30,
 		"grain_mass": 5.21,
-		"water_mass": 22
+		"water_mass": 22,
+		"volumen": 30
 	},
 	"default": {
 		"step": {

sude/GuiTest_no_user.json

@@ -7,7 +7,8 @@
 		"L": 0.2,
 		"Td": 30,
 		"grain_mass": 5.21,
-		"water_mass": 22
+		"water_mass": 22,
+		"volumen": 30
 	},
 	"default": {
 		"step": {

sude/sud_0010.json

@@ -7,7 +7,8 @@
 		"L": 0.2,
 		"Td": 30,
 		"grain_mass": 0,
-		"water_mass": 22
+		"water_mass": 22,
+		"volumen": 30
 	},
 	"default": {
 		"step": {

sude/sud_0010_downloaded.json

@@ -7,7 +7,8 @@
 		"L": 0.2,
 		"Td": 30,
 		"grain_mass": 0,
-		"water_mass": 22
+		"water_mass": 22,
+		"volumen": 30
 	},
 	"default": {
 		"step": {

sude/sud_0020.json

@@ -7,7 +7,8 @@
 		"L": 0.2,
 		"Td": 30,
 		"grain_mass": 0,
-		"water_mass": 10
+		"water_mass": 10,
+		"volumen": 30
 	},
 	"default": {
 		"step": {