Rename Sud's Download/Upload to Save/Load throughout

Renames everywhere the concept appears: Sud.download()/upload() ->
save()/load(), the 'Download'/'Upload' websocket messages ->
'Save'/'Load', the brewpi_gui menu actions, handlers and
sud_download_path -> sud_save_path, and the demo script. Behavior is
unchanged - this is naming only, from the client's perspective (save
to / load from a local file) rather than the server's.

scripts/demos/sud/demo_sud_save_load.py

@@ -0,0 +1,45 @@
+import json
+import shutil
+import tempfile
+from pathlib import Path
+
+from components.sud import Sud, SudState
+
+# Console-only walkthrough of Sud.save()/load() (no plot - there's no
+# numeric data here, just JSON plumbing). Runs against a scratch copy of a
+# real sud.json so sude/sud_0010.json itself is never touched.
+
+if __name__ == '__main__':
+	with tempfile.TemporaryDirectory() as tmp:
+		path = Path(tmp) / "sud.json"
+		shutil.copy("sude/sud_0010.json", path)
+
+		sud = Sud(str(path))
+		print(f"Loaded '{sud.name}', {len(sud.schedule)} steps")
+
+		doc = sud.save()
+		assert doc['Name'] == sud.name
+		print("save() returned the on-disk document")
+
+		sud.state = SudState.RAMPING
+		assert sud.load(doc) is False
+		print("load() refused while RAMPING")
+
+		sud.state = SudState.IDLE
+		doc['Name'] = 'Sud-Renamed'
+		doc['steps'][0]['descr'] = 'Edited step'
+		assert sud.load(doc) is True
+		assert sud.name == 'Sud-Renamed'
+		assert sud.schedule[0]['descr'] == 'Edited step'
+		assert sud.state == SudState.IDLE
+		print("load() applied the edit and reset to IDLE")
+
+		on_disk = json.loads(path.read_text())
+		assert on_disk['Name'] == 'Sud-Renamed'
+		print("load() persisted the edit to disk")
+
+		assert sud.load({'Name': 'broken'}) is False
+		assert sud.name == 'Sud-Renamed'
+		print("load() rejected malformed data, left schedule untouched")
+
+	print("All sud save/load checks passed")