docs: document the launch selectors and translate them

Add an Usage section to the README covering --account, --thread and
--message, the hex-only thread id, Qt's own options, the exit status 2 on a
bad option, and the single-instance hand-off over the local socket. Add the
Unreleased changelog entries for the selectors, the second-launch behaviour
and the plain-text status bar.

Translate the new strings into Italian. The option names stay untranslated in
the help text: they are wire format, and a translated option name is an option
the user cannot type.

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

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