hw_wallet: fix crash on exit if device unpairing fails

[sashazykov]

Quitting Electrum with a hardware wallet open crashes the process (SIGABRT) if the paired device was unplugged during the session.

HW_PluginBase.close_wallet unpaired the device before stopping the keystore TaskThread. Unpairing closes the hardware client, which does device I/O and raises if the device is gone:

E | plugin | Plugin error. plugin: trezor, hook: close_wallet
Traceback (most recent call last):
  File "electrum/plugin.py", line 833, in run_hook
    r = f(*args)
  File "electrum/hw_wallet/plugin.py", line 89, in close_wallet
    self.device_manager().unpair_pairing_code(keystore.pairing_code())
  File "electrum/plugin.py", line 1118, in unpair_pairing_code
    self._close_client(_id)
  File "electrum/plugin.py", line 1134, in _close_client
    client.close()
  ...
  File "electrum/plugins/trezor/clientbase.py", line 286, in close
    self.client.lock()
  ...
trezorlib.transport.bridge.BridgeException: trezord: acquire/62/null failed with code 400: device not found

run_hook() swallows the exception, so keystore.thread.stop() never runs. The still-running TaskThread is a QObject child of the wallet window, so at interpreter shutdown PyQt's atexit cleanup destroys the window and Qt aborts:

QThread: Destroyed while thread '' is still running

Fix

• hw_wallet/plugin.py: stop the keystore thread before unpairing, so a device-I/O failure cannot leak a running thread. This ordering is also safer in itself: previously the client could be closed while a queued task was still using it.
• plugin.py: DeviceMgr._close_client treats client.close() as best-effort and logs failures — a missing device is a normal condition during cleanup, and this also covers the other unpair callers (disconnect events, session timeouts).

Testing

• Regression tests added in tests/plugins/test_hw_wallet.py.
• Reproduced and verified on macOS with a Trezor One (open hw wallet → pair → unplug → quit): before, the traceback above followed by the Qt fatal and abort; after, a clean shutdown with the bridge failure logged:
  plugin.DeviceMgr | failed to close hardware client cleanly: BridgeException('trezord: acquire/64/null failed with code 400: device not found')