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/sagikazarmark/locafero/justfile

@@ -0,0 +1,14 @@
+default:
+    just --list
+
+test:
+    go test -count 10 -shuffle on -race -v ./...
+
+fuzz:
+    go test -race -v -fuzz=Fuzz -fuzztime=60s ./...
+
+lint:
+    golangci-lint run
+
+fmt:
+    golangci-lint fmt