feat: migrate login page to design system styling

Replace inline styles with CSS module matching the design system's
LoginForm visual patterns. Uses proper DS class structure (divider,
social section, form fields) while keeping username-based auth
instead of the DS component's email validation.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/auth/LoginPage.module.css

@@ -0,0 +1,85 @@
+.page {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  min-height: 100vh;
+  background: var(--bg-base);
+}
+
+.card {
+  width: 100%;
+  max-width: 400px;
+  padding: 32px;
+}
+
+.loginForm {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  font-family: var(--font-body);
+  width: 100%;
+}
+
+.logo {
+  margin-bottom: 8px;
+  font-size: 24px;
+  font-weight: 700;
+  color: var(--text-primary);
+}
+
+.subtitle {
+  font-size: 13px;
+  color: var(--text-muted);
+  margin: 0 0 24px;
+}
+
+.error {
+  width: 100%;
+  margin-bottom: 16px;
+}
+
+.socialSection {
+  display: flex;
+  flex-direction: column;
+  gap: 8px;
+  width: 100%;
+  margin-bottom: 20px;
+}
+
+.divider {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  width: 100%;
+  margin-bottom: 20px;
+}
+
+.dividerLine {
+  flex: 1;
+  height: 1px;
+  background: var(--border);
+}
+
+.dividerText {
+  color: var(--text-muted);
+  font-size: 11px;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+  font-weight: 500;
+}
+
+.fields {
+  display: flex;
+  flex-direction: column;
+  gap: 14px;
+  width: 100%;
+}
+
+.submitButton {
+  width: 100%;
+}
+
+.ssoButton {
+  width: 100%;
+  justify-content: center;
+}