Felix #7: handleMentionClick unconditionally preventDefault'd and goto'd,
breaking ctrl-click / cmd-click / shift-click / alt-click / middle-click —
"open in new tab" is a real workflow for researchers comparing two persons.
Add isPlainPrimaryClick() guard. Modified clicks fall through to the
browser's default anchor handling (the <a href="/persons/{id}"> opens in
the new tab as expected). Plain left-clicks still SPA-navigate via goto().
Three new tests assert ctrl-click, meta-click, and middle-click are not
preventDefault'd.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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