Switch from bundled subtrees to Composer dependencies

Replace the git-subtree vendored copies of txthinking/mailer and
psr/log with Composer-managed dependencies under vendor/, loaded
automatically

Switch from bundled subtrees to Composer dependencies

Replace the git-subtree vendored copies of txthinking/mailer and
psr/log with Composer-managed dependencies under vendor/, loaded
automatically by DokuWiki's plugin autoloader. The custom PSR-4
loader.php is no longer needed and has been removed.

Also modernize the plugin to current DokuWiki conventions:
- namespaced base classes, short array syntax, PSR code style
- rebuild the admin test form with dokuwiki\Form\Form
- key recipients by address in Message::getTo() so RCPT TO works
with the mailer's key-based iteration
- guard the Bcc continuation loop against running past the body
- avoid an undefined-key warning in getEHLO() on CLI sends
- migrate tests to the *Test.php naming and test namespace
- replace Travis with the shared DokuWiki GitHub Action

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