The supporter email's button opens a page on the site

[McCal-Codes]

What this changes

The supporter email's button was folio://redeem?c=<code> and did nothing in Gmail on Android (McCal tapped it on a phone, 2 Oct): Gmail won't open a custom-scheme link. It now links to https://foliolauncher.com/redeem/#<code>, a page on the site (McCal-Codes/folio-site#19) that shows the code and has an Add to Folio button that opens Folio's own redeem screen on Android, a Copy button, and the paste-it-in instructions. The code sits after the #, so it never reaches a server.

• Button label is "Add to Folio" (was "Open in Folio") and the line under it says "Opens a page that adds the code in Folio on your Android phone." That caption is mine, not yours.
• The plain-text email carries the same https link.
• tools/kofi-worker/email.js only; the worker's other behavior and the pool rules are unchanged.

How I checked it

• node --test in tools/kofi-worker: 36 pass. The link test now checks the https form, that no folio:// link is left in either part, and that the code in the fragment is something the page accepts (letters, digits and hyphens, 20 to 200).
• Looked at the email in the Mockup Lab in light and dark.
• Not deployed, and do not deploy before folio-site#19 is merged and live, or the button goes to a page that does not exist. Order: merge folio-site#19, check https://foliolauncher.com/redeem/#<anything-24-groups-long> shows a code, then merge this and run wrangler deploy (yours to run).
• Not checked: tapping the button from Gmail on a phone. That is the test after both are live.

Cheap way out: revert; wrangler rollback puts the earlier email back.

• No GPL code and nothing that needs root
• No app change, so no CHANGELOG line (REL-7 skipped)