Offer to pip install missing plugin dependencies instead of just warning

Builds on the previous branch's dependency check: the missing-deps
QMessageBox is now a Yes/No prompt. Yes runs a blocking
`sys.executable -m pip install <reqs>` (subprocess.run, output
captured), shows a result dialog, and on success proceeds straight to
enabling the plugin — no need to click Enable a second time.

Blocking is a deliberate simplification, not an oversight: this
codebase has no worker-thread/progress-dialog pattern for slow
operations anywhere else, so a threaded installer would be
inconsistent with everything else here. pip install is a one-time,
infrequent action, unlike e.g. a serial connect that runs constantly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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