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/spf13/cast/.golangci.yaml

@@ -0,0 +1,39 @@
+version: "2"
+
+run:
+  timeout: 10m
+
+linters:
+  enable:
+    - errcheck
+    - govet
+    - ineffassign
+    - misspell
+    - nolintlint
+    # - revive
+    - unused
+
+  disable:
+    - staticcheck
+
+  settings:
+    misspell:
+      locale: US
+    nolintlint:
+      allow-unused: false # report any unused nolint directives
+      require-specific: false # don't require nolint directives to be specific about which linter is being skipped
+
+formatters:
+  enable:
+    - gci
+    - gofmt
+    # - gofumpt
+    - goimports
+    # - golines
+
+  settings:
+    gci:
+      sections:
+        - standard
+        - default
+        - localmodule