[FEATURE]: Zen dashboard - usage totals, per-model breakdown, and invoices

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Describe the enhancement you want to request

The Zen usage history at opencode.ai/workspace/... currently shows individual API requests with their model, token counts, and cost. This is useful but missing some basics:

• Usage totals: there's no way to see total spend for a given period (day/week/month). You have to mentally sum up rows of $0.0000 entries.
• Per-model breakdown - no aggregation by model. If I'm using minimax-m2.5, glm-4.7, and claude-sonnet-4-5 in the same period, I'd like to see how much each cost me.
• Invoices - there's no way to download or view invoices. This is a blocker for anyone who needs to expense or account for Zen usage.

Even a simple summary table at the top (total input tokens, total output tokens, total cost, grouped by model and time period) would be very helpful

Thank you!