feat(apple): native SwiftUI app for iOS and macOS

Add a native SwiftUI VniDrop app (Send/Receive/Settings) talking to the
Rust core via generated UniFFI Swift bindings, plus the uniffi-bindgen
helper crate. iOS uses a TabView, macOS a NavigationSplitView sidebar.

apple/VniDrop/Features/Send/FilePreviewRepository.swift

@@ -0,0 +1,142 @@
+import Foundation
+import Combine
+
+/// Persisted per-transfer thumbnail store, ported from
+/// `feature/send/FilePreviewRepository.kt` + `PlatformPreviewStore.ios.kt`.
+/// Only small PNG/JPEG/WEBP previews are retained, under a total quota.
+struct PreviewStoragePolicy {
+	var maxEntryBytes: Int = 512 * 1024
+	var maxTotalBytes: Int64 = 20 * 1024 * 1024
+}
+
+private struct PreviewFileInfo {
+	let transferId: UInt64
+	let byteSize: Int64
+	let modifiedAtMillis: Int64
+}
+
+@MainActor
+final class FilePreviewRepository: ObservableObject {
+	@Published private(set) var previews: [UInt64: Data] = [:]
+
+	private let directory: String
+	private let policy: PreviewStoragePolicy
+	private let fm = FileManager.default
+
+	init(appDataDir: String, policy: PreviewStoragePolicy = PreviewStoragePolicy()) {
+		self.directory = (appDataDir as NSString).appendingPathComponent("ui/previews")
+		self.policy = policy
+	}
+
+	func restore(activeTransferIds: Set<UInt64>) {
+		let files = listFiles()
+		for file in files where !(activeTransferIds.contains(file.transferId)
+			&& (1...Int64(policy.maxEntryBytes)).contains(file.byteSize)) {
+			deleteFile(file.transferId)
+		}
+		enforceQuota()
+		var result: [UInt64: Data] = [:]
+		for file in listFiles() where activeTransferIds.contains(file.transferId) {
+			if let data = readFile(file.transferId), data.isSupportedPreview, data.count <= policy.maxEntryBytes {
+				result[file.transferId] = data
+			} else {
+				deleteFile(file.transferId)
+			}
+		}
+		previews = result
+	}
+
+	func save(transferId: UInt64, bytes: Data) {
+		guard (1...policy.maxEntryBytes).contains(bytes.count), bytes.isSupportedPreview else { return }
+		guard writeAtomically(transferId: transferId, bytes: bytes) else { return }
+		enforceQuota(protectedTransferId: transferId)
+		if readFile(transferId) != nil {
+			previews[transferId] = bytes
+		}
+	}
+
+	func remove(transferId: UInt64) {
+		deleteFile(transferId)
+		previews.removeValue(forKey: transferId)
+	}
+
+	// MARK: - Store (ported from IosPreviewStore)
+
+	private func enforceQuota(protectedTransferId: UInt64? = nil) {
+		let files = listFiles().sorted { $0.modifiedAtMillis < $1.modifiedAtMillis }
+		var total = files.reduce(Int64(0)) { $0 + $1.byteSize }
+		for file in files {
+			if total <= policy.maxTotalBytes { break }
+			if file.transferId == protectedTransferId { continue }
+			deleteFile(file.transferId)
+			total -= file.byteSize
+			previews.removeValue(forKey: file.transferId)
+		}
+	}
+
+	private func ensureDirectory() {
+		try? fm.createDirectory(atPath: directory, withIntermediateDirectories: true)
+	}
+
+	private func path(_ transferId: UInt64) -> String {
+		(directory as NSString).appendingPathComponent("\(transferId).preview")
+	}
+
+	private func listFiles() -> [PreviewFileInfo] {
+		ensureDirectory()
+		guard let names = try? fm.contentsOfDirectory(atPath: directory) else { return [] }
+		return names.compactMap { name in
+			guard name.hasSuffix(".preview"),
+				let id = UInt64(name.replacingOccurrences(of: ".preview", with: "")) else { return nil }
+			let full = (directory as NSString).appendingPathComponent(name)
+			guard let attrs = try? fm.attributesOfItem(atPath: full) else { return nil }
+			let size = (attrs[.size] as? NSNumber)?.int64Value ?? 0
+			let modified = ((attrs[.modificationDate] as? Date)?.timeIntervalSince1970 ?? 0) * 1000
+			return PreviewFileInfo(transferId: id, byteSize: size, modifiedAtMillis: Int64(modified))
+		}
+	}
+
+	private func readFile(_ transferId: UInt64) -> Data? {
+		try? Data(contentsOf: URL(fileURLWithPath: path(transferId)))
+	}
+
+	private func writeAtomically(transferId: UInt64, bytes: Data) -> Bool {
+		ensureDirectory()
+		if fm.fileExists(atPath: path(transferId)) { return true }
+		let temporary = (directory as NSString).appendingPathComponent(".\(transferId).tmp")
+		do {
+			try bytes.write(to: URL(fileURLWithPath: temporary), options: .atomic)
+		} catch {
+			return false
+		}
+		do {
+			if fm.fileExists(atPath: path(transferId)) {
+				try? fm.removeItem(atPath: temporary)
+				return true
+			}
+			try fm.moveItem(atPath: temporary, toPath: path(transferId))
+			return true
+		} catch {
+			try? fm.removeItem(atPath: temporary)
+			return false
+		}
+	}
+
+	private func deleteFile(_ transferId: UInt64) {
+		try? fm.removeItem(atPath: path(transferId))
+	}
+}
+
+extension Data {
+	/// Matches `isSupportedPreview()`: PNG / JPEG / WEBP magic bytes.
+	var isSupportedPreview: Bool {
+		let bytes = [UInt8](self)
+		let png = bytes.count >= 8 && bytes[0] == 0x89
+			&& bytes[1] == 0x50 && bytes[2] == 0x4E && bytes[3] == 0x47   // "PNG"
+		let jpeg = bytes.count >= 3 && bytes[0] == 0xFF && bytes[1] == 0xD8 && bytes[2] == 0xFF
+		let webp = bytes.count >= 12
+			&& bytes[0] == 0x52 && bytes[1] == 0x49 && bytes[2] == 0x46 && bytes[3] == 0x46   // "RIFF"
+			&& bytes[8] == 0x57 && bytes[9] == 0x45 && bytes[10] == 0x42 && bytes[11] == 0x50 // "WEBP"
+		return png || jpeg || webp
+	}
+}