Replace Use now with a plain Reconnect, and ignore superseded tests

Three review rounds kept finding ways Use now could point at an address a
reconnect wouldn't pick: the page was predicting the outcome of the server's
race from test results that could be unfinished, from another network, or out
of date after a reorder or a Tailscale change. Adding the offered LAN address
first in the list is what makes it win; Use now only hurried that along.

The dialog's Retry button now also shows while connected, as Reconnect: it tries
the addresses again in their current order and promises nothing about which
answers first. The test results go back to plain rows.

The review also found a test started earlier could overwrite a newer one still
running, and mark it done. Each headset's tests now have a generation; only the
newest one publishes.

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

docs/devices.md

@@ -78,11 +78,11 @@ can add, edit, reorder and remove them without leaving the page you're on. When
 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. When the test
-finishes and the first address that passed would be tried before the one in use,
-that address gets **Use now**, which reconnects. A reconnect tries the addresses
-in order and normally lands on it, but takes a later one if it's slow to answer
-then. Otherwise the change takes effect the next time Frame Control connects.
+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.
 
 ## Networks