Add "Cached Tokens" widget and metrics

[ndaidong]

I've been using Bifrost and considering GoModel as a replacement.
I prefer GoModel's minimalist UI but miss Bifrost's cached-token stats.

Proposal:

• In Usage Overview, add a new widget "Cached Tokens".
• In Daily Token Usage chart, add a new line for total cached tokens
• In Usage Analytics / Token Usage by Model, add new column for cached tokens (table mode)

[SantiagoDePolonia]

Thank you for the feedback! Definitely GoModel is going in this direction. The local caching and prompt caching visibility/observability will be increased soon. Once done, I'll update you here.