Make "check on launch" actually check on launch

The switch has been in Settings since the updater landed and has never
been read by anything: there was no launch-time check to turn on or off.
It is now wired to one.

It speaks only when there is a build to install. An app that is already
current says nothing, and neither does one that could not reach GitHub —
opening the app is not asking a question, and a phone with no signal
should not be told off for it once a launch. The button in Settings *is*
a question and still answers every time, up to date and failures alike.
That rule is `launchOffer`, next to the channel rule and tested with it.

Two things it is careful about:

The setting is read straight from the store rather than from the state
the screens are drawn with. That state begins as the defaults — where
checking is on — and is replaced a moment later by what is stored, so a
check driven off it would have fired on the default before ever learning
it had been switched off.

The prompt waits for the splash to clear before appearing, while the
check itself runs behind it. The look costs no time and cannot land on
top of the tartan.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01DCuRESmSZeJoJond43itw2

README.md

@@ -97,6 +97,12 @@ it: every rule above is a pure function and unit-tested on the JVM.
   builds from `dev`. The settings screen states which one is running rather than
   offering a switch.
 
+  The check on launch **only ever speaks when there is a build to install**. It
+  runs behind the splash and waits for it to clear, and an app that is already
+  current — or a phone that cannot reach GitHub — says nothing, because opening
+  the app was not a question. The button in Settings is a question, so it
+  answers every time, including "you're on the latest" and whatever went wrong.
+
   Wanting both means installing both — they are separate apps and each updates
   itself, see [Both channels on one device](#both-channels-on-one-device).