gui: add a persisted user config; remember the last Sud file dialog dir

New client/user_config.py - a small JSON-backed key/value store at
~/.config/brewpi/gui.json for GUI preferences that should survive
restarts (distinct from the server's config.json and from sude/*.json
schedules). First use: Save/Load Sud's file dialogs now open in
whichever directory was last used, instead of always starting at cwd.

client/user_config.py

@@ -0,0 +1,27 @@
+import json
+from pathlib import Path
+
+DEFAULT_PATH = Path.home() / ".config" / "brewpi" / "gui.json"
+
+
+class UserConfig:
+	"""Small persisted key/value store for GUI preferences that should
+	survive across restarts (e.g. the last directory used for a file
+	dialog) - local user state, not version-controlled config."""
+
+	def __init__(self, path=DEFAULT_PATH):
+		self.path = path
+		self.data = {}
+		if self.path.exists():
+			try:
+				self.data = json.loads(self.path.read_text())
+			except (json.JSONDecodeError, OSError):
+				self.data = {}
+
+	def get(self, key, default=None):
+		return self.data.get(key, default)
+
+	def set(self, key, value):
+		self.data[key] = value
+		self.path.parent.mkdir(parents=True, exist_ok=True)
+		self.path.write_text(json.dumps(self.data, indent='\t'))