[client] Keep file drop out of the wasm build

File drop needs a local filesystem to deliver into and a receiver it can bind,
so wasm supports neither and never sets a manager. The code still linked in,
because the Engine reached filedrop through an untagged field, and with it the
net/http server and client. That put the wasm binary 0.97MiB over the 60MB
budget, on a main that had 0.34MiB of headroom left.

Split the feature on the js tag: the logic file and the OS dialer become !js,
a js no-op carries the start, stop and rebind entry points the engine calls on
its lifecycle paths, and a build-tagged alias types the manager so engine.go
and connect.go stay untagged and lose the import. iOS keeps the full feature:
the logic file is !js and the iOS dialer stays behind the ios tag.

Drops the wasm binary to 59.89MiB, with no filedrop symbol left in it.

client/internal/engine.go

@@ -40,7 +40,6 @@ import (
 	dnsconfig "github.com/netbirdio/netbird/client/internal/dns/config"
 	"github.com/netbirdio/netbird/client/internal/dnsfwd"
 	"github.com/netbirdio/netbird/client/internal/expose"
-	"github.com/netbirdio/netbird/client/internal/filedrop"
 	"github.com/netbirdio/netbird/client/internal/ingressgw"
 	"github.com/netbirdio/netbird/client/internal/lazyconn"
 	"github.com/netbirdio/netbird/client/internal/metrics"
@@ -191,7 +190,7 @@ type EngineServices struct {
 	UpdateManager  *updater.Manager
 	ClientMetrics  *metrics.ClientMetrics
 	MetricsCtx     context.Context
-	FileDrop       *filedrop.Manager
+	FileDrop       fileDropManager
 	// NetMgr gates the reconnection loops on OS-reported network
 	// availability; nil disables gating.
 	NetMgr *netevents.Manager
@@ -254,7 +253,7 @@ type Engine struct {
 
 	sshServer sshServer
 
-	fileDrop        *filedrop.Manager
+	fileDrop        fileDropManager
 	fileDropRunning bool
 	fileDropPort    uint16
 	overlayWait     overlayWaiter //nolint:unused // only read by the iOS overlay wait