Fix request timezone precedence during user enrichment

[roncodes]

Summary

• add shared IANA timezone helper
• preserve request/browser timezone before backend IP lookup timezone
• support whois.timezone and whois.time_zone.name candidates
• avoid falling back to application/server timezone when no valid timezone exists

Verification

• php -l src/Support/Timezone.php src/Models/User.php tests/Unit/TimezoneTest.php
• vendor/bin/php-cs-fixer fix -v --dry-run --using-cache=no src/Models/User.php
• vendor/bin/php-cs-fixer fix -v --dry-run --using-cache=no src/Support/Timezone.php
• vendor/bin/php-cs-fixer fix -v --dry-run --using-cache=no tests/Unit/TimezoneTest.php
• vendor/bin/pest --colors=always tests/Unit/TimezoneTest.php

Note: Pest passed, but emitted an existing sandbox warning writing vendor/pestphp/pest/.temp/test-results.