fix: deploy keys modal crash on null/undefined arrays

- sshmanager/deploy.go: initialize DeployResult with empty slices
  instead of nil to prevent null serialization in JSON
- Machines.tsx: use ?? [] fallback for messages and errors arrays
  in deploy keys modal to handle null/undefined gracefully

web/src/pages/Machines.tsx

@@ -660,7 +660,7 @@ export default function Machines() {
                   <div className="rounded-card bg-emerald-500/10 border border-emerald-500/30 p-4">
                     <p className="text-sm font-medium text-emerald-400 mb-2">Deployment successful</p>
                     <ul className="space-y-1">
-                      {deployModal.result.messages.map((msg, i) => (
+                      {(deployModal.result.messages ?? []).map((msg, i) => (
                         <li key={i} className="text-xs text-fg-muted flex items-start gap-2">
                           <span className="text-emerald-400 mt-0.5">&#10003;</span>
                           {msg}
@@ -672,7 +672,7 @@ export default function Machines() {
                   <div className="rounded-card bg-rose-500/10 border border-rose-500/30 p-4">
                     <p className="text-sm font-medium text-rose-400 mb-2">Deployment failed</p>
                     <ul className="space-y-1">
-                      {deployModal.result.errors.map((err, i) => (
+                      {(deployModal.result.errors ?? []).map((err, i) => (
                         <li key={i} className="text-xs text-fg-muted flex items-start gap-2">
                           <span className="text-rose-400 mt-0.5">&#10007;</span>
                           {err}