Fix: Point recruit dialog share URL to Special:CreateAccount with UTM attribution

The shared recruitment message linked to the wiki's main page, giving no clear entry point for new users and producing no attribution signal for measuring referral activity from the Recruit editors & native speakers feature.

Point the URL in the shared message to Special:CreateAccount with utm_source=starterkit, and append utm_medium={platform} per share button so WhatsApp, Telegram, and Facebook shares can be measured independently. The textarea preview shows the base attribution URL (without utm_medium) so users editing the message before sharing see a clean URL; the correct platform medium is substituted at share-time — and stamped as facebook on the copy-message button in the Facebook row.

• RecruitDialog.vue: import wikiCode from the store; compute createAccountBaseUrl (https://{wikiCode}.wikipedia.org/wiki/Special:CreateAccount ?utm_source=starterkit) and a createAccountUrl(platform) helper that appends &utm_medium={platform}. share() substitutes the base URL with the platform-specific one before encoding. copyMessage() stamps the copied text with utm_medium=facebook.
• i18n/en.json: tweak message text to "Create a free account and join us here: 👉 {url}", matching the proposed wording in the task description.
• i18n/qqq.json: update $2 documentation to describe the new Special:CreateAccount URL with UTM attribution.

Co-Authored-By: Claude Opus 4.6

Bug: T431353