Offer Use now only where a reconnect goes, and keep newer edits and headsets apart

From the second independent review of this PR:

- Use now sends a plain reconnect, which picks the first-ranked address that
  answers, but it was offered on every tested address listed above the one in
  use. The devices list now carries each address's rank on the current network
  (frame_devices.order_addresses), and only the address a reconnect would pick
  gets the button.
- Saving one address, cancelling, then editing another: the first save's answer
  closed the second editor and lost what was typed. Each edit now has its own
  session, and a late answer leaves a newer one alone.
- Switching headsets with the dialog open drew the address offer from the
  previous headset's status before it was cleared, so Add could save its IP to
  the new headset. The dialog now renders after the old status is cleared.

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

docs/devices.md

@@ -78,9 +78,10 @@ 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 a tested
-address ranks above the one in use, **Use now** reconnects through it; otherwise
-the change takes effect the next time Frame Control connects.
+name still leads. A new or edited address is tested straight away. When an address
+that would be tried before the one in use passes the test, it gets **Use now**,
+which reconnects; the reconnect picks the first address that answers, which is
+that one. Otherwise the change takes effect the next time Frame Control connects.
 
 ## Networks