feat: accept the launch selectors on the command line

Connect first, become the server only if that fails. A live instance is handed
the selectors and this process exits without ever opening a database, so two
launches no longer mean two notmuch handles. When no instance answers and no
socket can be created either, the window still opens: losing single-instance
behaviour is a degradation, losing the mail client is not.

--version and --help stay hand-checked before QApplication exists: Qt's own
versions exit through QCoreApplication, and --version has to work where the
GUI cannot open. The selectors themselves are parsed after QApplication, from
app.arguments(), because the constructor strips Qt's standard options such as
-platform and -style; parsing the raw argv would reject them as unknown. An
unknown option prints the error on stderr and exits with status 2 before any
window is shown.

The socket's parent directory is created first, since the state directory does
not exist on a first run and listen() cannot create a socket inside a missing
directory.

Raising under Wayland is a request rather than a command. The compositor may
honour it as a focus hint or ignore it by policy; the selectors apply either
way, which is the half that has to work.

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

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