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/github.com/dustin/go-humanize/humanize.go

@@ -0,0 +1,8 @@
+/*
+Package humanize converts boring ugly numbers to human-friendly strings and back.
+
+Durations can be turned into strings such as "3 days ago", numbers
+representing sizes like 82854982 into useful strings like, "83 MB" or
+"79 MiB" (whichever you prefer).
+*/
+package humanize