fix(console): stop the dashboard before the shutdown message is drawn

[Bugs5382]

What and why

Since #366 the console shows a branded "Rebooting..." or "Shutting down..."
message as soon as a shutdown is chosen. The dashboard is a separate
supervised process (cmd/cryptos-console) writing to the same /dev/console
on its own refresh tick, and nothing told it to stop, so a tick landing after
the message was written could redraw the ordinary status frame over it (or
mix the two) until the screen went dark.

shutdownRequests.Wait now stops the console dashboard, and waits for it to
have actually exited, before announcing the shutdown message, so no dashboard
frame can land after it. run.go wires this by giving the console supervisor
its own cancelable context instead of sharing the node's: canceling it kills
the running cryptos-console child and stops the supervisor from relaunching
it, and stopConsole blocks until that has happened.

No docs change: the README already describes the branded screen as the
intended behavior; this fixes a race that could violate it, not the described
behavior itself.

Refs #370

Verification

• Lint clean
• Tests pass
• Build succeeds
• Documentation updated (if behavior or API changed) -- not needed, see above

How this was verified

• go build ./...
• systemd-run --user --scope -q -p MemoryMax=6G -p MemorySwapMax=0 go test ./... (all packages pass)
• golangci-lint run ./internal/init/... (0 issues)
• gofmt -l . (clean)
• New test TestShutdownRequests_WaitStopsTheConsoleBeforeAnnouncing fails without the fix
  (stopConsole undefined, then wrong call order) and passes with it, asserting stopConsole runs
  before announce.

[Bugs5382]

Approved. The dashboard stops before the shutdown message is drawn. Refs #370