fix: thread config.json's Pot.mass into both UI clients' displayed mass

Sud._parse_data() already falls back to config.json's Pot section when
a loaded sud.json doesn't override 'mass', but neither client actually
passed that config through, so displayed pot mass silently read 0 kg
whenever a recipe didn't set its own 'pot.mass'.

client/brewpi_gui.py: capture the System message's 'Pot' key (already
sent by the server, previously ignored) into self.pot_config, and pass
it to Sud._parse_data(doc, self.pot_config).

web/app.js: parseSudDoc()'s potMass now falls back to the module-level
potConfig.mass (already captured from msg.Pot elsewhere, just never
consulted here) before defaulting to 0.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M2ierBoxW3v7nUbDw3M2pE

web/app.js

@@ -153,7 +153,10 @@ function parseSudDoc(doc) {
 	return {
 		name: doc.Name || '',
 		schedule: schedule,
-		potMass:   pot.mass        || 0,
+		// Falls back to config.json's Pot.mass (via potConfig, set from
+		// the System message) when the loaded doc doesn't override it -
+		// mirrors components/sud.py's Sud._parse_data().
+		potMass:   pot.mass        || potConfig.mass || 0,
 		grainMass: pot.grain_mass  || 0,
 		waterMass: pot.water_mass  || 0,
 		volumen:   pot.volumen     || 0,