fix(mail): keep '&' intact in mailto links with multiple query params

Move the email-handling helpers (obfuscate, mail_isvalid,
mail_quotedprintable_encode, mail_setup) out of the procedural
inc/mai

fix(mail): keep '&' intact in mailto links with multiple query params

Move the email-handling helpers (obfuscate, mail_isvalid,
mail_quotedprintable_encode, mail_setup) out of the procedural
inc/mail.php into a namespaced dokuwiki\MailUtils class plus a new
Mailer::configInit(), and add a separate MailUtils::obfuscateUrl() for
the mailto-href context.

The xhtml renderer and PluginTrait now build the link label and the
href separately: the address half is run through the mailguard
obfuscation, the query string is preserved verbatim with only HTML
escaping applied. This fixes #1690 — in 'visible' mode the previous
code rawurlencoded the entire address+query, turning '?' into '%3F' and
breaking multi-parameter mailto links; in all modes the query string is
no longer mangled by the [at]/[dot] substitution.

Core call sites (Mailer, auth, LegacyApiCore, common, the xhtml
renderer, the parser, the bundled config/styling/usermanager plugins)
are migrated to MailUtils directly. The old top-level functions and
PREG_PATTERN_VALID_EMAIL constant remain as deprecated shims with
rector mappings.

Tests for obfuscate / mail_isvalid / mail_quotedprintable_encode are
consolidated into a single _test/tests/MailUtilsTest.php and extended
with regression coverage for the multi-parameter, double-escape and
URL-shape cases.

Closes #1690
Replaces #1964

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