Switch from HTTP basic auth to MCIAS

Replace HTTP basic auth with MCIAS session cookies. Adds mcdsl auth,
csrf, and web packages. Chi router for route-level auth middleware.
Short code redirects (GET /{short}) remain public; all management
pages require MCIAS login. Nord dark theme, layout+page template
pattern matching other platform services.

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

templates/list.html

@@ -0,0 +1,27 @@
+{{define "title"}} — All Links{{end}}
+{{define "content"}}
+<h2>All Links</h2>
+<p><a href="/">Shorten a URL</a></p>
+{{if .URLs}}
+<table>
+    <thead>
+        <tr>
+            <th>Short</th>
+            <th>URL</th>
+            <th>Created</th>
+        </tr>
+    </thead>
+    <tbody>
+        {{range .URLs}}
+        <tr>
+            <td><a href="/{{.Short}}">{{.Short}}</a></td>
+            <td><a href="{{.URL}}">{{.NURL}}</a></td>
+            <td>{{.Timestamp}}</td>
+        </tr>
+        {{end}}
+    </tbody>
+</table>
+{{else}}
+<p>No links yet.</p>
+{{end}}
+{{end}}