Show toasts above open dialogs, and harden the superseded-test test

From the review of the Reconnect change:

- A toast raised while a dialog was open (Reconnect refused during an install,
  say) sat under the dialog's backdrop, dimmed. The toast is now a manual
  popover, raised again when a dialog has opened since, so it shows on top.
- The new test could wait for ever if the first test never reached its probe,
  and didn't check its threads finished. It now waits on events with time
  limits, releases everything in finally, and also checks the first test
  finishing doesn't mark the second, still running, done.
- docs/devices.md: Reconnect applies your changes by reconnecting; which
  address wins still depends on ranking and timing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/devices.md

@@ -79,10 +79,9 @@ the headset reports a LAN IP on the same network as this computer and that IP
 isn't saved, it offers to add it. The offer puts the address first in the list,
 so on that network it wins over the Tailscale name; away from home the Tailscale
 name still leads. A new or edited address is tested straight away. While
-connected, **Reconnect** tries the addresses again in their current order, so an
-address just added or moved up is used now rather than at the next connection.
-It doesn't pick a particular address: the first to answer in order wins, and a
-slow one loses to a later one.
+connected, **Reconnect** applies your changes now rather than at the next
+connection: it tries the addresses again, ranked as above, and the best-ranked
+one that answers promptly wins. It doesn't pick a particular address.
 
 ## Networks