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>

go.sum

@@ -2,6 +2,8 @@ git.wntrmute.dev/kyle/goutils v1.21.0 h1:ZR7ovV400hsF09zc8tkdHs6vyen8TDJ7flong/d
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 git.wntrmute.dev/mc/mcdsl v1.4.0/go.mod h1:MhYahIu7Sg53lE2zpQ20nlrsoNRjQzOJBAlCmom2wJc=
+git.wntrmute.dev/mc/mcdsl v1.6.0 h1:Vn1uy6b1yZ4Y8fsl1+kLucVprrFKlQ4SN2cjUH/Eg2k=
+git.wntrmute.dev/mc/mcdsl v1.6.0/go.mod h1:MhYahIu7Sg53lE2zpQ20nlrsoNRjQzOJBAlCmom2wJc=
 github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
 github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
 github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=