feat: catch-all page for stopped apps, deploy controls, dashboard open button

Stopped/undeployed app subdomains now show a branded page instead of
Traefik 404. Deploy settings page gains start/stop/restart controls.
Dashboard shows "Megnyitás" button for running apps.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/web/templates/stacks.html

@@ -25,7 +25,7 @@
                 <div>
                     <h3>{{.Meta.DisplayName}}</h3>
                     {{$subdomain := index $.Subdomains .Name}}
-                    {{if $subdomain}}
+                    {{if and $subdomain .Deployed}}
                     <a class="subdomain-link" href="https://{{$subdomain}}.{{$.Domain}}" target="_blank">
                         {{$subdomain}}.{{$.Domain}} ↗
                     </a>