[MM-70642] Add Global Relay SenderAddress config setting (#38451)

* [MM-70642] Add Global Relay SenderAddress config setting

Add an optional SenderAddress to GlobalRelayMessageExportSettings so
compliance exports can use a fixed From / SMTP envelope sender for
Proofpoint and Cloud relay verification. Empty keeps today's
participant-derived From. Includes System Console UI, validation,
docs, and tests.

Co-authored-by: Joram Wilander <joram@mattermost.com>

* [MM-70642] Validate SenderAddress with IsValidEmail

Use model.IsValidEmail for GlobalRelaySettings.SenderAddress instead of
a substring "@" check, and cover malformed addresses in unit tests.

Co-authored-by: Joram Wilander <joram@mattermost.com>

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Co-authored-by: Cursor Agent <cursoragent@cursor.com>