Mail: Don't crash on invalid/missing From Email header

During testing, I found that some Email clients/providers format the
From header field content with a lower case f, so `from:` instead of
`From:`. Our client previously gave up if it couldn't find one that
starts with a capital F. Now, we try both, and provide a fallback if
neither were found.

Userland/Applications/Mail/MailWidget.cpp

@@ -368,10 +368,17 @@ void MailWidget::selected_mailbox()
 
  auto& from_iterator_value = from_iterator->get<1>().value();
  auto from_index = from_iterator_value.find("From:"sv);
- VERIFY(from_index.has_value());
- auto potential_from = from_iterator_value.substring(from_index.value());
- auto from_parts = potential_from.split_limit(':', 2);
- auto from = parse_and_unfold(from_parts.last());
+ if (!from_index.has_value())
+ from_index = from_iterator_value.find("from:"sv);
+ DeprecatedString from;
+ if (from_index.has_value()) {
+ auto potential_from = from_iterator_value.substring(from_index.value());
+ auto from_parts = potential_from.split_limit(':', 2);
+ from = parse_and_unfold(from_parts.last());
+ }
+
+ if (from.is_empty())
+ from = "(Unknown sender)";
 
  InboxEntry inbox_entry { from, subject };