SafeDrop v1.7.1

Community contribution — interface-aware desktop discovery. Thanks to
@JianBin0218 (PR #1).

Fixed

• Desktop discovery now computes each interface's directed broadcast
  from its real netmask instead of assuming /24. The previous /24
  guess produced the wrong broadcast on non-/24 LANs — most notably an
  iPhone Personal Hotspot (172.20.10.x is a /28, broadcast
  172.20.10.15), so phones on a hotspot never discovered the desktop.
  Now uses psutil to enumerate active, up interfaces and derive the
  broadcast per interface (safedrop/discovery.py).
• Multi-homed machines send each directed broadcast out the right
  interface. _broadcast_endpoints() pairs each broadcast target
  with its source IP and binds the send socket to that source, so a
  box with e.g. a Radmin/VPN adapter + Wi-Fi no longer leaks the
  Wi-Fi broadcast onto the VPN interface (or vice-versa).
• The status bar's local-IP display now lists every active interface
  IP (_local_ip_display()).

Changed

• New runtime dependency: psutil>=5.9 (interface enumeration).
• pyproject.toml → 1.7.1.

Tests

• tests/test_discovery.py rewritten around the new
  BroadcastInterface model: netmask-derived broadcast (incl. the /28
  hotspot case), down-adapter skipping, per-source-IP endpoint binding,
  multi-interface IP display. 104 / 104 green.

Note: this release is desktop-Python only. The iOS native discovery
already computes per-interface broadcasts from netmask via
getifaddrs (shipped in 1.7.0), so it was already correct for the
hotspot case.