Remove legacy default-server page, unify on multi-server registry

Deletes /wg-server (route, handlers, template) entirely and adds generic
/servers/:id/interface and /servers/:id/keypair endpoints + UI in the
All Servers page, so every server (including the default one) is managed
through the same per-server registry. Drops the write-through dual-write
hacks that kept the old single-server collection in sync - the registry
is now the single source of truth. One-time legacy-install migration path
is untouched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

store/store.go

@@ -11,13 +11,10 @@ type IStore interface {
 	SaveUser(user model.User) error
 	DeleteUser(username string) error
 	GetGlobalSettings() (model.GlobalSetting, error)
-	GetServer() (model.Server, error)
 	GetClients(hasQRCode bool) ([]model.ClientData, error)
 	GetClientByID(clientID string, qrCode model.QRCodeSettings) (model.ClientData, error)
 	SaveClient(client model.Client) error
 	DeleteClient(clientID string) error
-	SaveServerInterface(serverInterface model.ServerInterface) error
-	SaveServerKeyPair(serverKeyPair model.ServerKeypair) error
 	SaveGlobalSettings(globalSettings model.GlobalSetting) error
 	GetWakeOnLanHosts() ([]model.WakeOnLanHost, error)
 	GetWakeOnLanHost(macAddress string) (*model.WakeOnLanHost, error)