feat(web): add Discord login button, platform indicator, and CSS styles

- Discord login button on /login with brand colors
- NavMenu shows user avatar (Discord) and platform label
- CSS: login-divider, login-btn-discord, nav-user-info, nav-user-platform
- NavMenu version bumped to v2.8.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Web/wwwroot/app.css

@@ -1619,3 +1619,78 @@ body.telegram-mini-app .session-card-mobile {
         display: flex;
     }
 }
+
+/* === Discord Login Button === */
+.login-btn-discord {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    gap: 0.75rem;
+    width: 100%;
+    padding: 0.75rem 1.25rem;
+    border-radius: var(--radius-md);
+    font-weight: 500;
+    font-size: 0.9375rem;
+    text-decoration: none;
+    transition: all 0.2s ease;
+    background-color: #5865F2;
+    color: white;
+    border: none;
+    cursor: pointer;
+}
+
+.login-btn-discord:hover {
+    background-color: #4752C4;
+    transform: translateY(-1px);
+}
+
+.login-btn-discord:active {
+    transform: translateY(0);
+}
+
+.login-btn-discord .login-btn-icon {
+    width: 20px;
+    height: 20px;
+    flex-shrink: 0;
+}
+
+.login-divider {
+    display: flex;
+    align-items: center;
+    margin: 1.25rem 0;
+    color: var(--text-muted);
+    font-size: 0.875rem;
+}
+
+.login-divider::before,
+.login-divider::after {
+    content: "";
+    flex: 1;
+    height: 1px;
+    background: var(--border-color);
+}
+
+.login-divider span {
+    padding: 0 1rem;
+}
+
+/* === Platform Indicator in Nav === */
+.nav-user-info {
+    display: flex;
+    flex-direction: column;
+    gap: 0.125rem;
+}
+
+.nav-user-platform {
+    font-size: 0.6875rem;
+    color: var(--text-muted);
+    text-transform: uppercase;
+    letter-spacing: 0.025em;
+}
+
+.nav-user-avatar img {
+    width: 100%;
+    height: 100%;
+    object-fit: cover;
+    border-radius: 50%;
+}