feat: add policy deletion, improve dry run handling, and enhance policy designer UX

Add DELETE /policies/{policy_id} endpoint with audit logging, improve firewall apply to handle dry run mode without calling provider and track operation success separately from applied status, update Proxmox provider error message to clarify rule-to-VM mapping requirement, add dry run explanation text to FirewallPreview with conditional button labels, enhance Policies page with expanded DataTable columns showing source

backend/app/services/providers/proxmox.py

@@ -130,4 +130,8 @@ class ProxmoxProvider(Provider):
     async def apply_rules(self, connection: ProviderConnection, rules: list[dict[str, Any]]) -> dict[str, Any]:
         if connection.read_only:
             return {"applied": False, "reason": "Cluster is read-only", "rules": rules}
-        return {"applied": False, "reason": "Apply adapter intentionally requires explicit implementation", "rules": rules}
+        return {
+            "applied": False,
+            "reason": "Live Proxmox firewall apply needs rule-to-VM mapping before NexaFabric can safely write provider rules.",
+            "rules": rules,
+        }