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/dashboard.html

@@ -174,6 +174,8 @@
                 <a href="/stacks/{{.Name}}/deploy" class="btn btn-sm btn-primary" onclick="return checkBeforeDeploy(event, '{{.Name}}')">Telepítés</a>
             {{else}}
                 {{if isOperational .State}}
+                    {{$subdomain := index $.Subdomains .Name}}
+                    {{if $subdomain}}<a href="https://{{$subdomain}}.{{$.Domain}}" target="_blank" class="btn btn-sm btn-outline" onclick="event.stopPropagation()">Megnyitás ↗</a>{{end}}
                     <button class="btn btn-sm btn-warning" onclick="stackAction(event, '{{.Name}}', 'restart')">↻</button>
                     <button class="btn btn-sm btn-danger" onclick="stackAction(event, '{{.Name}}', 'stop')">■</button>
                 {{else}}