Custom unsubscribe message for SMS campaigns

Problem: The current unsubscribe toggle only appends a hardcoded "Reply STOP to unsubscribed." This works for two-way SMS services but fails for one-way SMS, which many users rely on.

In several countries in Europe the opt-out instruction must reference a specific short code (e.g. "Reply STOP to 36180"), similar to Brevo's [STOP_CODE] system where

the keyword STOP_CODE will be automatically replaced with a code specific to SMS campaigns or transactional SMS messages when the message is sent.

Proposed solution: Add an optional free-text field below the existing unsubscribe toggle in Campaign > Appearance. When populated, it overrides the default message. When left empty, the current default behavior is preserved.

Ideas:

• New text area field appears under the unsubscribe toggle (only visible when toggle is ON)

• Field is optional — empty = default message used

• Custom message is appended to SMS body in place of the default unsubscribe line

• Per-campaign setting (saved with campaign blueprint)

• Works with both one-way and two-way SMS services

Priority: High — this is a legal compliance blocker for users sending SMS in countries that require carrier-specific STOP codes.