Call the person who follows an External interaction's link the link holder, not the visitor

Problem/Motivation

The module calls the person who follows an External interaction's link "the visitor", in its interface, its documentation and its code. Everybody on a website is a visitor, so the word says nothing about this person, and it suggests someone from outside the organization, which they need not be.

What sets them apart is how they get in: through the link, without signing in. The link is their only credential, whoever they are.

Proposed resolution

• Say "link holder" where a person must be named.
• Elsewhere, speak of the link itself: "the page the link opens", "the step the link opens", "when the link is followed".
• Apply it to the interface strings and their French translations, the documentation, code comments, and test names and helpers.

Only wording changes: no stored configuration or data uses the word.

Remaining tasks

• Reword the 17 interface strings and their translations.
• Reword the documentation and code comments.
• Rename the test methods and helpers that say "visitor".

User interface changes

Interface texts say "link holder", or speak of the link, instead of "visitor".

API changes

None.

Data model changes

None.

AI-Generated: Yes (Claude Code was used to help draft this issue summary. I reviewed it.)