Tell the invited reader plainly how logging in works

[JeroenDeDauw]

The invitation's instruction sentence becomes "You will receive a one-time code via email each
time. There is no password.", in both the formatted part and the plain one. The no-password
sentence stays: a reader who assumes a password heads for the password reset, which deliberately
mails members nothing.

Follows-up to #30

_{AI-authored — Claude Code, Fable 5 (max); exact wording from @JeroenDeDauw, one revision (dash dropped); diff not yet human-reviewed; full extension suite run locally against the change, nothing pins the sentence.}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.58%. Comparing base (b439052) to head (f17d543).

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #32   +/-   ##
=========================================
  Coverage     86.58%   86.58%           
  Complexity      677      677           
=========================================
  Files            75       75           
  Lines          2378     2378           
=========================================
  Hits           2059     2059           
  Misses          319      319           

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