fix(routing+backends): Pfad-Präfix-Validierung + Warnung bei leerem Backend-Pool (v1.1.141)

- RoutingRules: path_prefix Pattern-Validator (muss mit / beginnen) + destroyOnHidden
- Backends: Alert-Banner wenn aktives Backend 0 Server hat (→ HAProxy 503)

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

management-ui/src/i18n/locales/en/common.json

@@ -380,6 +380,8 @@
     "forceHttp1Hint": "Disables HTTP/2 (h2) on the backend connection — HAProxy negotiates HTTP/1.1 only. Required for backends that don't support h2 (e.g. older nginx configs without the h2 module, legacy apps).",
     "servers": "Servers",
     "noServers": "no server",
+    "noServersWarning": "One or more active backends have no servers configured.",
+    "noServersWarningDesc": "HAProxy will return 503 for any traffic routed to these backends. Open the backend (orange tag) and add at least one server.",
     "nServers": "{{n}} servers",
     "serversIn": "Servers in “{{name}}”",
     "serverHintCreate": "Saving creates the pool only. Add servers in the next step — open the pool and click “Add server”.",
@@ -415,6 +417,8 @@
     "editRule": "Edit rule",
     "domain": "Domain",
     "pathPrefix": "Path prefix",
+    "pathPrefixHint": "Must start with /. HAProxy uses prefix matching.",
+    "pathPrefixInvalid": "Path must start with / and contain only valid URL characters",
     "backend": "Backend",
     "priority": "Priority",
     "active": "Active",