[client] Port the file drop bindings to iOS

The iOS SDK package gets the same platform handle the Android bindings
carry: transfers, receiving policy, and per-sender rules on one profile,
attached to the engine when the client starts. Payloads are path-based
rather than stream-based, since the app group container is readable from
both processes, and the platform sink is dropped for the same reason:
delivery lands on the filesystem directly. The default destination is
seeded in the constructor because the app process opens handles of its
own while the extension is down.

client/ios/NetBirdSDK/filedrop_transfer.go

@@ -0,0 +1,164 @@
+//go:build ios
+
+package NetBirdSDK
+
+import (
+	"strings"
+	"time"
+
+	"github.com/netbirdio/netbird/client/internal/filedrop"
+)
+
+// The file drop receiving modes exported via gomobile.
+const (
+	FileDropModeOff        = int(filedrop.ModeOff)
+	FileDropModeAsk        = int(filedrop.ModeAsk)
+	FileDropModeAutoAccept = int(filedrop.ModeAutoAccept)
+)
+
+// The per-sender rules exported via gomobile.
+const (
+	FileDropRuleDefault      = int(filedrop.SenderRuleDefault)
+	FileDropRuleAlwaysAccept = int(filedrop.SenderRuleAlwaysAccept)
+	FileDropRuleBlock        = int(filedrop.SenderRuleBlock)
+)
+
+// The transfer states exported via gomobile.
+const (
+	FileDropStatePending      = int(filedrop.StatePending)
+	FileDropStateTransferring = int(filedrop.StateTransferring)
+	FileDropStateCompleted    = int(filedrop.StateCompleted)
+	FileDropStateDeclined     = int(filedrop.StateDeclined)
+	FileDropStateExpired      = int(filedrop.StateExpired)
+	FileDropStateCancelled    = int(filedrop.StateCancelled)
+	FileDropStateFailed       = int(filedrop.StateFailed)
+)
+
+// The failure reasons exported via gomobile.
+const (
+	FileDropReasonNone        = int(filedrop.ReasonNone)
+	FileDropReasonUnreachable = int(filedrop.ReasonUnreachable)
+)
+
+// The event kinds delivered to a FileDropListener.
+const (
+	FileDropEventOffer     = int(filedrop.EventOffer)
+	FileDropEventCompleted = int(filedrop.EventCompleted)
+	FileDropEventFailed    = int(filedrop.EventFailed)
+	FileDropEventWithdrawn = int(filedrop.EventWithdrawn)
+	FileDropEventProgress  = int(filedrop.EventProgress)
+)
+
+// FileDropListener receives transfer events. Calls arrive on background
+// goroutines, so implementations must post to the UI thread themselves.
+type FileDropListener interface {
+	OnFileDropEvent(kind int, transfer *FileDropTransfer)
+}
+
+// FileDropFile is one item of a transfer.
+type FileDropFile struct {
+	Name        string
+	Size        int64
+	ContentType string
+	IsText      bool
+	Text        string
+}
+
+// FileDropTransfer is one history entry.
+type FileDropTransfer struct {
+	ID          string
+	Outgoing    bool
+	PeerKey     string
+	PeerName    string
+	State       int
+	Transferred int64
+	TotalSize   int64
+	// Unix milliseconds, so the platform layer can render the time in the
+	// user's own locale and zone rather than parsing a preformatted string.
+	CreatedAtMillis int64
+	UpdatedAtMillis int64
+	// IsText marks a transfer that is a single inline snippet rather than
+	// files, so the UI can drop the size and offer a copy action instead.
+	IsText bool
+	Error  string
+	Reason int
+
+	files          []*FileDropFile
+	deliveredPaths []string
+}
+
+// FileDropTransferArray wraps transfers for gomobile compatibility.
+type FileDropTransferArray struct {
+	items []*FileDropTransfer
+}
+
+// FileCount returns the number of items in the transfer.
+func (t *FileDropTransfer) FileCount() int {
+	return len(t.files)
+}
+
+// GetFile returns the item at index i, or nil when out of range.
+func (t *FileDropTransfer) GetFile(i int) *FileDropFile {
+	if i < 0 || i >= len(t.files) {
+		return nil
+	}
+	return t.files[i]
+}
+
+// DeliveredPaths returns the delivered file paths joined by newlines, so the
+// platform layer can move them into user-visible storage.
+func (t *FileDropTransfer) DeliveredPaths() string {
+	return strings.Join(t.deliveredPaths, "\n")
+}
+
+// Length returns the number of transfers.
+func (a *FileDropTransferArray) Length() int {
+	return len(a.items)
+}
+
+// Get returns the transfer at index i, or nil when out of range.
+func (a *FileDropTransferArray) Get(i int) *FileDropTransfer {
+	if i < 0 || i >= len(a.items) {
+		return nil
+	}
+	return a.items[i]
+}
+
+func toFileDropTransfer(t filedrop.Transfer) *FileDropTransfer {
+	files := make([]*FileDropFile, 0, len(t.Files))
+	for _, f := range t.Files {
+		files = append(files, &FileDropFile{
+			Name:        f.Name,
+			Size:        f.Size,
+			ContentType: f.ContentType,
+			IsText:      f.Kind == filedrop.KindText,
+			Text:        f.Text,
+		})
+	}
+
+	return &FileDropTransfer{
+		ID:              string(t.ID),
+		Outgoing:        t.Direction == filedrop.DirectionSent,
+		PeerKey:         string(t.PeerKey),
+		PeerName:        t.PeerName,
+		State:           int(t.State),
+		Transferred:     t.Transferred,
+		TotalSize:       t.TotalSize,
+		CreatedAtMillis: unixMillis(t.CreatedAt),
+		UpdatedAtMillis: unixMillis(t.UpdatedAt),
+		IsText:          len(t.Files) == 1 && t.Files[0].Kind == filedrop.KindText,
+		Error:           t.Error,
+		Reason:          int(t.Reason),
+		files:           files,
+		deliveredPaths:  t.DeliveredPaths,
+	}
+}
+
+// unixMillis renders a timestamp for the platform layer, mapping the zero time
+// to 0 so it reads as "unknown" rather than as 1970.
+func unixMillis(t time.Time) int64 {
+	if t.IsZero() {
+		return 0
+	}
+	return t.UnixMilli()
+}