fix(migration): detect flat structure in upsDevices for proper v3→v4 migration

The previous migration only checked for upsList field, but saveConfig()
strips upsList when saving, creating a race condition. If the daemon
restarted with a partially-migrated config (upsDevices with flat structure),
the migration wouldn't run because it only looked for upsList.

Now shouldRun() also detects:
- upsDevices with flat structure (host at top level, no snmp object)

And migrate() handles both:
- config.upsList (pure v3)
- config.upsDevices with flat structure (partially migrated)

This fixes the "Cannot read properties of undefined (reading 'host')"
error that occurred when configs had upsDevices but flat structure.

deno.json

@@ -1,6 +1,6 @@
 {
   "name": "@serve.zone/nupst",
-  "version": "4.0.4",
+  "version": "4.0.5",
   "exports": "./mod.ts",
   "tasks": {
     "dev": "deno run --allow-all mod.ts",