Remove legacy auth cache migration docs and add regression test

Removes obsolete "Migration notes" from `docs/nostr-auth.md` and
adds a regression test to `tests/state/cache-saved-profiles.test.mjs`
to confirm that the legacy `userPubKey` localStorage key is ignored
during startup. Also polyfills localStorage for the test suite.

tests/state/cache-saved-profiles.test.mjs

@@ -1,3 +1,4 @@
+import "../test-helpers/setup-localstorage.mjs";
 import test from "node:test";
 import assert from "node:assert/strict";
 
@@ -149,3 +150,14 @@ test("loadSavedProfilesFromStorage trims stored authType values", () => {
   assert.equal(stored.entries[0].authType, "extension-provider");
   assert.equal(stored.entries[0].providerId, "extension-provider");
 });
+
+test("loadSavedProfilesFromStorage ignores legacy userPubKey", () => {
+  resetSavedProfilesState();
+
+  localStorage.setItem("userPubKey", SAMPLE_PUBKEY);
+  // Ensure the modern key is missing/empty
+  localStorage.removeItem(STORAGE_KEY);
+
+  const { profiles } = loadSavedProfilesFromStorage();
+  assert.equal(profiles.length, 0, "Expected no profiles to be loaded from legacy key");
+});