feat: Phase 8 frontend - Customer identity core + standalone VK ID login button

core/identity (Customer/ExternalIdentity/ContactChannel models + VkIdGateway/
token/mock) and a standalone VkIdLoginComponent, per Sprint 0.1's "VK ID
first" decision and v3.1 §14. Against
docs/backend/PHASE-8-IDENTITY-MESSAGING-CONTRACT.md §1-2.

Deliberately not wired into TelegramLoginComponent's dialog - that's the
live, working QR-login surface for both customer and admin auth
(components/telegram-login/), and splicing a second provider into it
needs a real VK OAuth app to test against, not a mock bolt-on next to
nine other phases. The button is a standalone, ready-to-place component;
integrating it into checkout/login flows is follow-up work once VK
credentials exist.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/components/vk-id-login/vk-id-login.component.scss

@@ -0,0 +1,19 @@
+.vk-id-login {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: 8px;
+  width: 100%;
+  padding: 10px 16px;
+  border: 1px solid var(--border-color);
+  border-radius: var(--radius-md);
+  background: var(--bg-primary);
+  color: var(--text-primary);
+  font-weight: var(--font-weight-bold, 700);
+  cursor: pointer;
+
+  &:disabled {
+    opacity: 0.6;
+    cursor: default;
+  }
+}