udisks2: mount/unmount without sudo, auto-mount before sync

- m key toggles mount/unmount of the selected device via udisksctl
  (resolves the block device from mount point or lsblk label)
- rsync-style device sync auto-mounts the target first
- mlove finalize unmounts via udisksctl, falling back to sudo -n umount
- keybar/help/README updated

src/ui/mod.rs

@@ -81,6 +81,7 @@ fn draw_help(f: &mut Frame) {
         "  ↑↓ / j k     move selection",
         "  s / Enter     sync selected device",
         "  a             abort running sync",
+        "  m             mount / unmount selected device (udisks)",
         "  Tab / 1-3     switch tab",
         "  r             refresh devices",
         "  f             toggle log follow",