Privatemsg emails include placeholder markup in URL

Problem/Motivation

Outgoing private message notification emails contain placeholder markup in the thread URL, for example:
You have received a new private message. To read your message, follow this link: http://www.example.com/messages/view/38909

Steps to reproduce

In a Drupal 10 environment where privatemsg is enabled, user A sends a private message to user B. Inspect the email that is received by user B.

Expected: clean clickable URL that goes to the relevant message
Actual: http://www.example.com/messages/view/38909

Proposed resolution

Before: URL built manually; notification used hard-coded 'ru'.
After: Generate link via Url::fromRoute(...); use $member->getPreferredLangcode() for recipient language.
Why: Ensures correct route building (aliases/language) and sends notifications in the recipient's preferred language. No API changes.

Remaining tasks

None (on my side)

User interface changes

None

API changes

None

Data model changes

None