build: package haul as a macOS .app bundle

Double-clicking the bare binary in Finder opens a Terminal window alongside
the app: Finder has no better idea what to do with a Mach-O executable than
hand it to Terminal.app, and the terminal stays up as long as the process
does. macOS has no counterpart to -H=windowsgui, so the only fix is to ship
an application bundle, which Finder launches directly.

Add `make package-darwin`, which runs a pinned `fyne package --target
darwin` to produce ./haul.app. Its metadata comes from cmd/haul/FyneApp.toml
— the app ID there must stay in step with the one app.NewWithID uses, or
macOS treats the bundle and the running app as separate applications. The
icon is a placeholder; `fyne package` will not run without one.

Note that `fyne package` rewrites FyneApp.toml in place on every run, so the
file is left in that tool's canonical formatting rather than a commented
version it would only strip again.

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

cmd/haul/FyneApp.toml

@@ -0,0 +1,14 @@
+Website = "https://gitea.apointless.space/bsncubed/haul"
+
+[Details]
+Icon = "Icon.png"
+Name = "haul"
+ID = "space.apointless.haul"
+Version = "0.1.0"
+Build = 1
+
+[LinuxAndBSD]
+GenericName = "SFTP Client"
+Categories = ["Network", "FileTransfer", "Utility"]
+Comment = "Two-pane SFTP file browser over SSH"
+Keywords = ["sftp", "ssh", "scp", "file transfer"]