Campaign web links were generated from MINIAPP_CUSTOM_URL which is often
empty, causing get_campaign_web_link() to return None. Admins and partners
could only share bot links for campaigns, not cabinet links.
Now prefers CABINET_URL (where the auth flow captures ?campaign= param),
falling back to MINIAPP_CUSTOM_URL for backwards compatibility. This is
consistent with how referral web links already use CABINET_URL.
Fringg
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