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/sqlite/builder.json

@@ -0,0 +1,6 @@
+{
+	"autogen": "<none>",
+	"autotag": "<none>",
+	"autoupdate": "<none>",
+	"test": "darwin/(amd64|arm64)|freebsd/(amd64|arm64)|linux/(386|amd64|arm|arm64|loong64|ppc64le|riscv64|s390x)|openbsd/(amd64|arm64)|windows/(amd64|arm64|386)"
+}