Surface Sud's Load rejection as a server-driven error, not GUI guesswork

Sud.load() already refuses a new schedule while a run is in progress
(state not IDLE/DONE) - but tasks/sud.py's SudTask.recv() silently
dropped that refusal, and an earlier attempt to handle it client-side
by pre-emptively greying out the File menu based on the GUI's own
(replicated, laggy) view of sud_state was reverted: the intelligence
belongs on the server, which already knows definitively whether it's
running, not on the client guessing from a mirrored state that can
lag or miss the message that changed it.

tasks/sud.py: when Sud.load() returns False, send {'Sud': {'Error':
...}} explaining why, instead of doing nothing.

client/brewpi_gui.py: New/Save/Load Sud stay enabled unconditionally;
on_sud_changed() shows whatever 'Error' the server sends via
QMessageBox.warning(). on_action_sud_new()'s comment updated to match
(it goes through the same Load path, so gets the same rejection/error
for free).

Verified live: triggering Load while a run is in progress on an
isolated test server round-trips the Error over the wire and pops the
expected dialog ("Cannot load a new schedule while a run is in
progress - stop it first.").

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LhiQe64F74uHV8jzuoSa5K

client/brewpi_gui.py

@@ -659,8 +659,11 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 		self.msg_sud.send({'Confirm': True})
 
 	def on_action_sud_new(self):
-		# Replaces the running Sud's schedule with an empty one, same as
-		# Load but with a built-in empty document instead of a file dialog.
+		# Sends an empty schedule via the same Load path Load Sud uses, with
+		# a built-in empty document instead of a file dialog - the server
+		# itself refuses this (like any Load) while a run is in progress,
+		# surfacing an error (see on_sud_changed()'s 'Error' handling)
+		# rather than the GUI trying to guess that here.
 		self.msg_sud.send({'Load': {'Name': '', 'Description': '', 'pot_mass': 0, 'pot_material': None, 'steps': []}})
 
 	def on_action_sud_save(self):
@@ -876,6 +879,8 @@ class Window(QtWidgets.QMainWindow, Ui_MainWindow):
 					self.server_remaining_forecast = (anchor_min, forecast['T'], forecast['Theta'])
 			elif "Forecast" in key:
 				self.on_sud_forecast_received(msg['Forecast'])
+			elif "Error" in key:
+				QtWidgets.QMessageBox.warning(self, "Sud", msg['Error'])
 		self.update_sud_actions()
 
 	def on_sud_forecast_received(self, forecast):