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}",
+};

lib/settings.js

@@ -0,0 +1,45 @@
+import { DEFAULT_SETTINGS, OPEN_IN_VALUES } from "./defaults.js";
+
+// Pure. Layers stored values over the defaults, dropping unknown keys and
+// falling back on wrong types or out-of-range values. A fresh or signed-out
+// profile returns {} from storage, which yields the defaults.
+export function mergeSettings(stored) {
+  const merged = { ...DEFAULT_SETTINGS };
+  if (stored === null || typeof stored !== "object") {
+    return merged;
+  }
+  if (typeof stored.model === "string") {
+    merged.model = stored.model;
+  }
+  if (typeof stored.temporaryChat === "boolean") {
+    merged.temporaryChat = stored.temporaryChat;
+  }
+  if (typeof stored.webSearch === "boolean") {
+    merged.webSearch = stored.webSearch;
+  }
+  if (OPEN_IN_VALUES.includes(stored.openIn)) {
+    merged.openIn = stored.openIn;
+  }
+  if (typeof stored.promptTemplate === "string") {
+    merged.promptTemplate = stored.promptTemplate;
+  }
+  return merged;
+}
+
+// storage.sync works with no Mozilla account (it behaves as local storage and
+// syncs later if the user signs in), so no capability probe is needed. Any
+// rejection falls back to defaults rather than blocking the user's action.
+// Firefox sanitises all non-quota backend errors to a single generic string, so
+// there is nothing useful to branch on.
+export async function load() {
+  try {
+    const stored = await browser.storage.sync.get(Object.keys(DEFAULT_SETTINGS));
+    return mergeSettings(stored);
+  } catch {
+    return mergeSettings(null);
+  }
+}
+
+export async function save(partial) {
+  await browser.storage.sync.set(partial);
+}