feat(profile): sign-in methods matrix — email add/change, provider link/unlink

Profile shows the account's sign-in methods for guests and durable accounts alike:
add or change email, link Telegram on the web (login widget), and unlink a linked
provider. Email is never unlinked (it is changed); unlink is offered only when
another identity remains, and a change to an address owned by another account shows
the non-disclosing 'check the address or contact support'. Add-VK-on-web stays
deferred (no VK OAuth).

Wires linkUnlink / changeEmailRequest / changeEmailConfirm through the client,
transport, mock and codec (encodeLinkUnlink + LinkResult 'unlinked'/'changed'
statuses); codec wire tests; ru/en i18n.

ui/src/lib/codec.test.ts

@@ -28,6 +28,7 @@ import {
   encodeExchange,
   encodeExportUrlRequest,
   encodeGuestLogin,
+  encodeLinkUnlink,
   encodeStateRequest,
   encodeSubmitPlay,
   encodeTarget,
@@ -458,6 +459,22 @@ describe('codec', () => {
     });
   });
 
+  it('encodes an unlink request carrying the provider kind', () => {
+    const r = fb.LinkUnlinkRequest.getRootAsLinkUnlinkRequest(new ByteBuffer(encodeLinkUnlink('telegram')));
+    expect(r.kind()).toBe('telegram');
+  });
+
+  it('passes an unlinked / changed LinkResult status straight through', () => {
+    for (const status of ['unlinked', 'changed'] as const) {
+      const b = new Builder(64);
+      const s = b.createString(status);
+      fb.LinkResult.startLinkResult(b);
+      fb.LinkResult.addStatus(b, s);
+      b.finish(fb.LinkResult.endLinkResult(b));
+      expect(decodeLinkResult(b.asUint8Array()).status).toBe(status);
+    }
+  });
+
   it('decodes a merged LinkResult carrying a switched session', () => {
     const b = new Builder(128);
     const token = b.createString('tok-9');