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/pages/admin/OidcAdminPage.tsx

@@ -194,7 +194,7 @@ function OidcAdminForm() {
             type="url"
             value={form.issuerUri}
             onChange={(e) => updateField('issuerUri', e.target.value)}
-            placeholder="https://auth.example.com/realms/main"
+            placeholder="https://auth.example.com/realms/main/.well-known/openid-configuration"
           />
         </div>
 

ui/src/pages/executions/SearchFilters.module.css

@@ -111,6 +111,7 @@
   outline: none;
   width: 180px;
   transition: border-color 0.2s;
+  color-scheme: light dark;
 }
 
 .dateInput:focus { border-color: var(--amber-dim); }