Send the login code as a formatted mail

[alistair3149]

The code arrived as five lines of unstyled text, with nothing in it to say which wiki had sent it or
that the eight digits in the middle were the point. It read like something to be deleted unread,
which is a poor start for the one message standing between a member and their account.

Send it as a formatted part with a plain-text alternative: the wiki's name, the code set apart in a
panel of its own, how long it lasts, and what to do if it was not you. The plain part carries the
same code and the same warning rather than a note that the mail is better read elsewhere, since for
many wikis it is the only part that ships.

The formatted part is a Mustache template, so what it looks like is changed by editing markup rather
than by concatenating strings in PHP. It is table layout and inline styles throughout, because a
mail client is not a browser and the classic Outlook still in use across the companies this
extension serves lays out with Word's engine. Every string reaches it already localised, and
Mustache escapes what it interpolates, so a wiki's site name cannot carry markup into the mail.

Nothing in it is fetched from the network. No link, since a login mail that trains its reader to
click one is a phishing template with a head start, and no image, since clients block remote ones by
default and would show a broken box where a logo went.

MediaWiki keeps the formatted part only where $wgAllowHTMLEmail is on, which it is not by default.
That setting governs every mail a wiki sends, so it is recommended in the README rather than set
here, which is what Echo does with it too. Without it the code still arrives, as the plain part
alone.

The code is shown in groups of four so it can be held in the head between the mail and the form.
That is a display alone: a member who copies the spaces along with the digits is no longer refused
their own code.

Considered, omitted:

• setting $wgAllowHTMLEmail from the extension, which would reformat every mail the wiki sends
• a logo, which clients block by default and which the wiki's name in type does the job of
• putting the code in the subject line, which is the biggest convenience in an OTP flow and also
  puts the code on a locked screen — left as it was, and worth deciding on its own

Verified by sending a real code through the dev wiki and reading both parts out of the mailbox,
rather than only in tests.

_{AI-authored — Claude Code, Opus 5 (1M context); design chosen by @alistair3149 from rendered options, who also settled the $wgAllowHTMLEmail question; diff not yet human-reviewed; PHPCS, PHPStan and 756 tests pass locally, and the delivered mail was rendered and inspected.}

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.75000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.75%. Comparing base (0c08453) to head (65f579b).

Files with missing lines	Patch %	Lines
src/Application/DisplayedCode.php	66.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #20      +/-   ##
============================================
+ Coverage     86.67%   86.75%   +0.07%     
- Complexity      567      574       +7     
============================================
  Files            63       64       +1     
  Lines          1959     1985      +26     
============================================
+ Hits           1698     1722      +24     
- Misses          261      263       +2     

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[alistair3149]

HTML	Plain text

We could have an option to allow the wiki logo in the email, but that would be a follow-up.