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/mathutil/sqr.go

@@ -0,0 +1,12 @@
+//go:build !riscv64 && !loong64
+// +build !riscv64,!loong64
+
+package mathutil
+
+import "github.com/remyoudompheng/bigfft"
+
+func (f *float) sqr() {
+	f.n = bigfft.Mul(f.n, f.n)
+	f.fracBits *= 2
+	f.normalize()
+}