Migrate to mcdsl: auth, config, csrf, web

- Replace internal/auth with mcdsl/auth
- Replace internal/config with mcdsl/config (embed config.Base)
- Replace internal/webserver/csrf.go with mcdsl/csrf
- Use mcdsl/web for session cookies and template rendering
- Use mcdsl/httpserver for server setup and StatusWriter
- Remove direct mcias client library dependency
- Update .golangci.yaml to v2 format (formatters section)

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

web/templates/dashboard.html

@@ -0,0 +1,19 @@
+{{define "title"}} - Dashboard{{end}}
+{{define "content"}}
+<div class="card">
+    <div class="card-title">Session</div>
+    <div class="success">Login successful. MCIAS accepted this service context.</div>
+    <dl class="session-info">
+        <dt>Username</dt>
+        <dd>{{.Username}}</dd>
+        <dt>Roles</dt>
+        <dd>{{range .Roles}}<span class="role-tag">{{.}}</span>{{end}}</dd>
+        <dt>Service Name</dt>
+        <dd><code>{{.ServiceName}}</code></dd>
+        {{if .Tags}}
+        <dt>Tags</dt>
+        <dd>{{range .Tags}}<code>{{.}}</code> {{end}}</dd>
+        {{end}}
+    </dl>
+</div>
+{{end}}