[client] preserve WireGuard key on interactive re-login (#6777)

NewAuth built a fresh in-memory config on every call via
CreateInMemoryConfig, which generates a new WireGuard private key when
none is set. The iOS Swift layer calls this on interactive re-login and
writes the resulting config back to the profile's netbird.cfg, so each
re-auth replaced the peer's persisted private key with a new one. A new
key means a new public key, so the management server registered a
brand-new peer on every re-authentication — named after the fallback
hostname.

Load the existing config with DirectUpdateOrCreateConfig when a config
file is already present so re-login reuses the peer's persisted private
key (and its identity). Only fall back to a fresh in-memory config for
the first-time login when no config file exists yet (or after logout,
which deletes the file). DirectUpdateOrCreateConfig uses non-atomic
writes so it also works inside the tvOS App Group sandbox. This matches
what Run() and LoginForMobile() already do.

## Describe your changes

## Issue ticket number and link

## Stack

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### Checklist
- [x] Is it a bug fix
- [ ] Is a typo/documentation fix
- [ ] Is a feature enhancement
- [ ] It is a refactor
- [ ] Created tests that fail without the change (if possible)
- [ ] This change does **not** modify the public API, gRPC protocols,
functionality behavior, CLI / service flags, or introduce a new feature
— **OR** I have discussed it with the NetBird team beforehand (link the
issue / Slack thread in the description). See
[CONTRIBUTING.md](https://github.com/netbirdio/netbird/blob/main/CONTRIBUTING.md#discuss-changes-with-the-netbird-team-first).

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## Documentation
Select exactly one:

- [ ] I added/updated documentation for this change
- [x] Documentation is **not needed** for this change (explain why)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added support for loading or creating persistent configuration when a
configuration file path is provided.
* Continued support for in-memory configuration for temporary or
first-time use.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

client/ios/NetBirdSDK/login.go

@@ -44,10 +44,25 @@ type Auth struct {
 // NewAuth instantiate Auth struct and validate the management URL
 func NewAuth(cfgPath string, mgmURL string) (*Auth, error) {
 	inputCfg := profilemanager.ConfigInput{
+		ConfigPath:    cfgPath,
 		ManagementURL: mgmURL,
 	}
 
-	cfg, err := profilemanager.CreateInMemoryConfig(inputCfg)
+	// Load the existing config when a config file is already present so an
+	// interactive re-login reuses the peer's persisted WireGuard private key
+	// (and thus its identity) instead of generating a fresh one. Generating a
+	// new key registers a brand-new peer on the management server on every
+	// re-auth (named after the fallback hostname). Only fall back to a fresh
+	// in-memory config for the first-time login when no config file exists yet.
+	// DirectUpdateOrCreateConfig uses non-atomic writes so it also works inside
+	// the tvOS App Group sandbox where atomic temp-file+rename is blocked.
+	var cfg *profilemanager.Config
+	var err error
+	if cfgPath != "" {
+		cfg, err = profilemanager.DirectUpdateOrCreateConfig(inputCfg)
+	} else {
+		cfg, err = profilemanager.CreateInMemoryConfig(inputCfg)
+	}
 	if err != nil {
 		return nil, err
 	}