Add SSO login support

- Add [sso] config section with redirect_uri
- Create mcdsl/sso client when SSO is configured
- Add /login (landing page), /sso/redirect, /sso/callback routes
- Add /logout route
- Update login template with SSO landing page variant
- Bump mcdsl to v1.6.0

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

vendor/modernc.org/libc/README.md

@@ -0,0 +1,9 @@
+# libc
+
+[![LiberaPay](https://liberapay.com/assets/widgets/donate.svg)](https://liberapay.com/jnml/donate)
+[![receives](https://img.shields.io/liberapay/receives/jnml.svg?logo=liberapay)](https://liberapay.com/jnml/donate)
+[![patrons](https://img.shields.io/liberapay/patrons/jnml.svg?logo=liberapay)](https://liberapay.com/jnml/donate)
+
+[![Go Reference](https://pkg.go.dev/badge/modernc.org/libc.svg)](https://pkg.go.dev/modernc.org/libc)
+
+Package libc is a partial reimplementation of C libc in pure Go.