Allow custom percentage splits for capacity allocation across channels within a queue, rather than only equal splits.

Customer request: Configure Admin Omni Support queue with 70% capacity for chat and 30% for email.

Current limitation: Advanced capacity splitting only supports equal splits (e.g., 50/50 for two channels).

Requested by: Canva (AG Vanguardia)

Also requested by: Bullhorn (70/30 split), Dialpad (80/20 phones/email split)