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/sourcegraph/conc/panics/try.go

@@ -0,0 +1,11 @@
+package panics
+
+// Try executes f, catching and returning any panic it might spawn.
+//
+// The recovered panic can be propagated with panic(), or handled as a normal error with
+// (*panics.Recovered).AsError().
+func Try(f func()) *Recovered {
+	var c Catcher
+	c.Try(f)
+	return c.Recovered()
+}