Name the address on the code screen, and offer another code

[alistair3149]

Follows-up to #17

The screen asking for the code said nothing about where it went, offered no way to ask for another,
and no way back to the address box. A visitor who mistyped their address had nothing to see it by
and nowhere to go.

Name the address back, from the authentication session rather than the form, and add a button that
sends another code to it and a link back to the start of the login form. Starting the form again
already asks for an address rather than resuming the code screen, so only the way to it was missing.

The address is named whatever the allowlist made of it, so the screen still says the same for an
admitted address and one it has never heard of.

Asking for another code goes through the same use case as the first, so it is counted by the same
throttle, and it replaces the handle the session holds, so the code sent before it stops working.

The code box becomes optional. Required, it would have to be filled before the button asking for
another code could be pressed, which is exactly what a visitor without a working code needs to do.
An empty box is answered on instead, without spending one of the code's attempts.

Considered, omitted:

• a countdown to when the code expires, which needs the issue time carried to the screen
• rate limiting resends separately from first requests — the existing per-address burst limit of 3
  in 15 minutes already bounds them

Based on #17 rather than master, since it builds on the field that PR introduces.

_{AI-authored — Claude Code, Opus 5 (1M context); three-point ask from @alistair3149, no revisions; diff not yet human-reviewed; PHPCS, PHPStan and 734 tests pass locally, and the flow was driven end to end over HTTP — request, resend, old code rejected, new code logs in — but not viewed in a browser.}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.23%. Comparing base (53a9db6) to head (2c1b26d).

Additional details and impacted files

@@                           Coverage Diff                            @@
##             separate-email-field-on-login-form      #19      +/-   ##
========================================================================
+ Coverage                                 84.88%   87.23%   +2.34%     
- Complexity                                  574      596      +22     
========================================================================
  Files                                        64       65       +1     
  Lines                                      1992     2100     +108     
========================================================================
+ Hits                                       1691     1832     +141     
+ Misses                                      301      268      -33     

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[alistair3149]