feat: add settings defaults and validation with unit tests

lib/defaults.js

@@ -0,0 +1,13 @@
+// Single source of truth for the settings shape and their defaults.
+// Defaults reproduce the extension's pre-1.1 behaviour exactly: an empty model
+// and both toggles off mean the only URL parameter sent is `q`.
+
+export const OPEN_IN_VALUES = ["new-tab", "background-tab", "new-window"];
+
+export const DEFAULT_SETTINGS = {
+  model: "",
+  temporaryChat: false,
+  webSearch: false,
+  openIn: "new-tab",
+  promptTemplate: "{query}",
+};