Add OIDC login flow to UI and fix dark mode datetime picker icons

- Add "Sign in with SSO" button on login page (shown when OIDC is configured)
- Add /oidc/callback route to exchange authorization code for JWT tokens
- Add loginWithOidcCode action to auth store
- Treat issuer URI as complete discovery URL (no auto-append of .well-known)
- Update admin page placeholder to show full discovery URL format
- Fix datetime picker calendar icon visibility in dark mode (color-scheme)

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

ui/src/auth/LoginPage.module.css

@@ -92,6 +92,48 @@
   cursor: not-allowed;
 }
 
+.ssoButton {
+  width: 100%;
+  padding: 10px 16px;
+  border-radius: var(--radius-sm);
+  border: 1px solid var(--border);
+  background: var(--bg-raised);
+  color: var(--text-primary);
+  font-family: var(--font-body);
+  font-size: 14px;
+  font-weight: 600;
+  cursor: pointer;
+  transition: all 0.15s;
+}
+
+.ssoButton:hover {
+  border-color: var(--amber-dim);
+  background: var(--bg-surface);
+}
+
+.ssoButton:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
+.divider {
+  position: relative;
+  text-align: center;
+  margin: 20px 0;
+  border-top: 1px solid var(--border-subtle);
+}
+
+.dividerText {
+  position: relative;
+  top: -0.65em;
+  padding: 0 12px;
+  background: var(--bg-surface);
+  font-size: 12px;
+  color: var(--text-muted);
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+}
+
 .error {
   margin-top: 12px;
   padding: 10px 12px;