Show separate charts and cost values for multiple configured Claude providers (do not combine them)

[Jardo-51]

Currently, if you have multiple Claude providers configured (work / personal), the usage page combines all of them into a single chart and cost label.

This is not really that useful. I need to track the usage for my work account separately. The usage page should show a separate cost label and chart for each configured Claude provider, not a combined label / chart.

Optionally, there could be a toggle to combine the multiple providers into one.

This feature was explicitly requested by discussion #6920, but it was closed as resolved by #11485. That PR reads the usage from all configured providers, but still combines them into a single value and chart, so the ask from the discussion was not really resolved.