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/viper/TROUBLESHOOTING.md

@@ -0,0 +1,32 @@
+# Troubleshooting
+
+## Unmarshaling doesn't work
+
+The most common reason for this issue is improper use of struct tags (eg. `yaml` or `json`). Viper uses [github.com/mitchellh/mapstructure](https://github.com/mitchellh/mapstructure) under the hood for unmarshaling values which uses `mapstructure` tags by default. Please refer to the library's documentation for using other struct tags.
+
+## Cannot find package
+
+Viper installation seems to fail a lot lately with the following (or a similar) error:
+
+```
+cannot find package "github.com/hashicorp/hcl/tree/hcl1" in any of:
+/usr/local/Cellar/go/1.15.7_1/libexec/src/github.com/hashicorp/hcl/tree/hcl1 (from $GOROOT)
+/Users/user/go/src/github.com/hashicorp/hcl/tree/hcl1 (from $GOPATH)
+```
+
+As the error message suggests, Go tries to look up dependencies in `GOPATH` mode (as it's commonly called) from the `GOPATH`.
+Viper opted to use [Go Modules](https://go.dev/wiki/Modules) to manage its dependencies. While in many cases the two methods are interchangeable, once a dependency releases new (major) versions, `GOPATH` mode is no longer able to decide which version to use, so it'll either use one that's already present or pick a version (usually the `master` branch).
+
+The solution is easy: switch to using Go Modules.
+Please refer to the [wiki](https://go.dev/wiki/Modules) on how to do that.
+
+**tl;dr* `export GO111MODULE=on`
+
+## Unquoted 'y' and 'n' characters get replaced with _true_ and _false_ when reading a YAML file
+
+This is a YAML 1.1 feature according to [go-yaml/yaml#740](https://github.com/go-yaml/yaml/issues/740).
+
+Potential solutions are:
+
+1. Quoting values resolved as boolean
+1. Upgrading to YAML v3 (for the time being this is possible by passing the `viper_yaml3` tag to your build)