Follow the system's default audio device when a new one is plugged in #160

Split out of #40.

Since 18034d7 tincan survives losing its device: the stream's error callback marks it lost and, once a second, tincan reopens the same device or falls back to the default. What it does not do is notice a better device arriving while the current one keeps working. Plug in a headset or connect Bluetooth headphones mid-call and tincan stays on the built-in speaker and microphone until you switch by hand.

Expected: when tincan is on the default device (nothing named on the command line, nothing remembered), a change of the system default moves it to the new default, the same way the lost-device path already reopens a stream. A device the user chose by name stays chosen.

Notes:

• cpal has no device-change notification, so this likely means comparing the default device's name in the existing once-a-second recover tick.
• The switch should say so on screen, as recovery already does.