Preview 8 (1cbeaa6)

fix: show connection problems only when they last, and fix bad UI patterns

Connections:

• retryNow() starts the count again, and the app calls it on Retry, when
  it comes to the foreground and when the network changes, so Retry shows
  a spinner at once and an old error does not come back on open.
• A dropped or refused connection shows as a problem once it lasts 3 s;
  a slow attempt shows when it ends; a TLS failure, a refusal or a server
  that is not Rex shows at once.
• A wake from before a connection no longer skips the wait after it.
• HostState.loaded: Home waits for the first views and pane facts, up to
  2 s, and keeps a host's sessions on screen during a short reconnect.

Home and other screens:

• A dot, spinner or ring before the host's name, and a line of text only
  for a host with a problem.
• New session, Tailscale sign-out, split and new tab show progress and
  run once at a time.
• Background alerts shows the saved choice, with Allow notifications when
  notifications are off.
• The host editor checks the address when the user leaves the field, and
  text fields take taps on their whole row.
• The Tailscale screen keeps the Rex servers it found while it is in the
  back stack, and shows no raw error text.
• Fonts start from the cache, so screens do not jump while they load.
• Screen reader labels for keys, sliders, theme tiles and pane chips.

Terminal:

• A message over the last screen takes the touches, turns the keys off
  and closes the keyboard.
• The key bar fits narrow phones, and pane chips show the user's names.
• A screen Rex cannot send shows a message with Retry, not a blank pane.
• Closing a pane moves to the next one from the current view.
• Dropped input, waiting tabs in the tab menu, a jump-to-bottom button,
  Sign in and Allow on the pane's message, and copy feedback before
  Android 13.
• A notification for the open session switches its screen to the pane.