fix(quick-261029-d9j): pick the mail language from Accept-Language, add mail.fallback_locale, Polish subjects

- preferred_locale and the request locale are normalised (comma lists,
  q-weights, region subtags, case) to pl or en by NormalizeMailLocale, so a
  raw Accept-Language such as en-GB,en;q=0.9 selects the -en templates
- new golem15.user.mail.fallback_locale (default pl) is used when neither
  names a supported language; ResolveMailLocaleFor applies it, and Boot
  rejects any value other than pl or en
- activate, reactivate and restore Polish templates get Polish subjects
- no template, route or payload changes; MailTemplate and
  ResolveMailLocale keep their signatures

config/config.yaml

@@ -15,6 +15,14 @@ activation:
   activate_mode: auto
   reset_ttl_minutes: 60
   activation_ttl_hours: 72
+mail:
+  # The language of this plugin's mails when neither the user's
+  # preferred_locale nor the request locale (the translate plugin's locale,
+  # else the Accept-Language header) names one the plugin has mail for (pl or
+  # en). pl keeps the unsuffixed Polish templates; an English-only site sets
+  # en (SUMMER_GOLEM15__USER__MAIL__FALLBACK_LOCALE=en). Any other value
+  # fails boot.
+  fallback_locale: pl
 registration:
   allow_registration: true
   use_register_throttle: true