Add SSO login support

MCAT can now redirect users to MCIAS for SSO login (including passkey
support) instead of showing its own login form. SSO is opt-in via the
[sso] config section.

- Add SSO landing page with "Sign in with MCIAS" button
- Add /sso/redirect and /sso/callback routes
- Update mcdsl to v1.5.0 (sso package)
- Fix .gitignore: /mcat ignores only the root binary, not cmd/mcat/
- Track cmd/mcat/ source files (previously gitignored by accident)

Security:
- State cookie uses SameSite=Lax for cross-site redirect compatibility
- Session cookie remains SameSite=Strict after login

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

go.sum

@@ -1,5 +1,5 @@
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-git.wntrmute.dev/mc/mcdsl v1.2.0/go.mod h1:lXYrAt74ZUix6rx9oVN8d2zH1YJoyp4uxPVKQ+SSxuM=
+git.wntrmute.dev/mc/mcdsl v1.5.0 h1:JUlSYuvETRCycf+cZ56Gxp/1XZn0T7fOfWezM3m89qE=
+git.wntrmute.dev/mc/mcdsl v1.5.0/go.mod h1:MhYahIu7Sg53lE2zpQ20nlrsoNRjQzOJBAlCmom2wJc=
 github.com/cpuguy83/go-md2man/v2 v2.0.6/go.mod h1:oOW0eioCTA6cOiMLiUPZOpcVxMig6NIQQ7OS05n1F4g=
 github.com/go-chi/chi/v5 v5.2.5 h1:Eg4myHZBjyvJmAFjFvWgrqDTXFyOzjj7YIm3L3mu6Ug=
 github.com/go-chi/chi/v5 v5.2.5/go.mod h1:X7Gx4mteadT3eDOMTsXzmI4/rwUpOwBHLpAfupzFJP0=