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/limits/capi_linux_386.go

@@ -0,0 +1,5 @@
+// Code generated by 'ccgo limits/gen.c -crt-import-path  -export-defines  -export-enums  -export-externs X -export-fields F -export-structs  -export-typedefs  -header -hide _OSSwapInt16,_OSSwapInt32,_OSSwapInt64 -o limits/limits_linux_386.go -pkgname limits', DO NOT EDIT.
+
+package limits
+
+var CAPI = map[string]struct{}{}