feat(send): build durable transfer details and sharing flow

shared/src/jvmMain/kotlin/com/vnidrop/app/feature/send/PlatformPreviewStore.jvm.kt

@@ -0,0 +1,22 @@
+package com.vnidrop.app.feature.send
+
+import java.io.File
+
+actual fun createPlatformPreviewStore(appDataDir: String): PlatformPreviewStore = JvmPreviewStore(File(appDataDir, "ui/previews"))
+
+private class JvmPreviewStore(private val directory: File) : PlatformPreviewStore {
+	override fun list(): List<PreviewFileInfo> = directory.listFiles().orEmpty().mapNotNull { file ->
+		file.name.removeSuffix(".preview").toULongOrNull()?.let { PreviewFileInfo(it, file.length(), file.lastModified()) }
+	}
+	override fun read(transferId: ULong): ByteArray? = runCatching { file(transferId).takeIf(File::isFile)?.readBytes() }.getOrNull()
+	override fun writeAtomically(transferId: ULong, bytes: ByteArray): Boolean = runCatching {
+		directory.mkdirs()
+		val target = file(transferId)
+		if (target.isFile) return@runCatching true
+		val temporary = File(directory, ".${target.name}.tmp")
+		temporary.writeBytes(bytes)
+		temporary.renameTo(target).also { if (!it) temporary.delete() }
+	}.getOrDefault(false)
+	override fun delete(transferId: ULong) { file(transferId).delete() }
+	private fun file(transferId: ULong) = File(directory, "$transferId.preview")
+}

shared/src/jvmMain/kotlin/com/vnidrop/app/feature/send/TransferShareActions.jvm.kt

@@ -0,0 +1,62 @@
+package com.vnidrop.app.feature.send
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import java.awt.EventQueue
+import java.awt.FileDialog
+import java.awt.Frame
+import java.awt.KeyboardFocusManager
+import java.io.File
+
+@Composable
+actual fun rememberTransferShareActions(): TransferShareActions = remember {
+	object : TransferShareActions {
+		override val canUseNativeShare = DesktopShareBridge.shareFile != null
+		override val nfcAvailability = NfcShareAvailability.Hidden
+
+		override fun exportInvitation(ticket: String, transferName: String, onResult: (Result<Unit>) -> Unit) {
+			EventQueue.invokeLater {
+				onResult(runCatching {
+					val dialog = FileDialog(activeFrame(), "Save VniDrop invitation", FileDialog.SAVE).apply {
+						file = invitationFileName(transferName)
+					}
+					try {
+						dialog.isVisible = true
+						val directory = dialog.directory
+						val name = dialog.file
+						if (directory != null && name != null) File(directory, name).writeText(ticket)
+					} finally { dialog.dispose() }
+				})
+			}
+		}
+
+		override fun shareInvitation(ticket: String, transferName: String, onResult: (Result<Unit>) -> Unit) {
+			EventQueue.invokeLater {
+				val share = DesktopShareBridge.shareFile
+				if (share == null) {
+					onResult(Result.failure(UnsupportedOperationException("System sharing is unavailable on this desktop")))
+					return@invokeLater
+				}
+				onResult(runCatching {
+					val directory = File(System.getProperty("java.io.tmpdir"), "vnidrop-share").apply { mkdirs() }
+					val file = File(directory, invitationFileName(transferName)).apply { writeText(ticket) }
+					share(file).getOrThrow()
+				})
+			}
+		}
+
+		override fun writeInvitationToNfc(ticket: String, onResult: (Result<Unit>) -> Unit) {
+			onResult(Result.failure(UnsupportedOperationException("NFC is unavailable on desktop")))
+		}
+		override fun cancelNfcWrite() = Unit
+	}
+}
+
+private fun activeFrame(): Frame? =
+	(KeyboardFocusManager.getCurrentKeyboardFocusManager().activeWindow as? Frame)
+		?: Frame.getFrames().firstOrNull { it.isActive || it.isFocused }
+
+object DesktopShareBridge {
+	@Volatile
+	var shareFile: ((File) -> Result<Unit>)? = null
+}