docs+test(email): document the confirm deeplink + wire-contract tests

Document the one-tap confirm deeplink in ARCHITECTURE (§4 the login magic-link /
link confirm+profile-refresh, prefetch-safe token) and the new notify 'profile'
sub-kind (§10), and add the one-tap link to the FUNCTIONAL email story (+ru). Add
codec round-trip assertions for the EmailRequest language field and
encodeEmailConfirmLink (the mock e2e bypasses the codec).

ui/src/lib/codec.test.ts

@@ -21,6 +21,7 @@ import {
   decodeStats,
   encodeCheckWord,
   encodeEmailRequest,
+  encodeEmailConfirmLink,
   encodeFeedbackSubmit,
   encodeDraftSave,
   encodeEnqueue,
@@ -121,6 +122,12 @@ describe('codec', () => {
     );
     expect(email.email()).toBe('a@example.com');
     expect(email.browserTz()).toBe('+00:00');
+    expect(email.language()).toBe('en');
+
+    const confirm = fb.EmailConfirmLinkRequest.getRootAsEmailConfirmLinkRequest(
+      new ByteBuffer(encodeEmailConfirmLink('tok-abc')),
+    );
+    expect(confirm.token()).toBe('tok-abc');
 
     const vk = fb.VKLoginRequest.getRootAsVKLoginRequest(
       new ByteBuffer(encodeVKLogin('vk_user_id=494075&vk_ts=1&sign=abc', '+03:00', 'Иван Петров')),