Offer "quit and update" in the footer, check for updates on every start

The update check ran at most once a day and only printed a line after the
TUI exited. Now the registry is asked on every start (in the background,
3 s timeout, no cache) and the result reaches the TUI as a message.

With a newer release, the footer shows `q quit and update to vX` right of
help and quit, on a highlighted background (Theme.Highlight). `q` quits
and `main` installs the release once the terminal is restored, sharing
the code of `submarine update`; plain quit moves to `Q`, ctrl+c never
updates.

Drop LatestCached and CheckInterval. Add D27 and F58.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/requirements.md

@@ -164,6 +164,12 @@ submarine [dir]     # the repository containing dir (default: current directory)
 submarine --mock    # built-in mock data
 ```
 
+- **F58** On every start the tool looks for a newer release in the
+  background (never blocking, silent on failure; off for dev builds,
+  `--mock` and `SUBMARINE_NO_UPDATE_CHECK=1`). If there is one, the footer
+  shows `q quit and update to vX` right of help and quit, highlighted; `q`
+  then quits and installs the release, and plain quit is `Q`.
+
 ## Non-functional requirements
 
 - **N1** Startup must not block on the network. The tool shows the local