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

views/mail/activate.htm

@@ -1,4 +1,4 @@
-subject = "Activate your account"
+subject = "Aktywuj konto"
 description = "Aktywacja konta"
 layout = "user"
 ==

views/mail/reactivate.htm

@@ -1,4 +1,4 @@
-subject = "Welcome back"
+subject = "Witaj ponownie"
 description = "Konto zostało ponownie aktywowane"
 layout = "user"
 ==

views/mail/restore.htm

@@ -1,4 +1,4 @@
-subject = "Reset your password"
+subject = "Ustaw nowe hasło"
 description = "Reset hasła"
 layout = "user"
 ==