Merge the two "by Model" pies (Token Usage + API Requests) into one
dual-pie card with a shared right-side vertical legend and per-donut
center totals, drop the standalone API-Key Top 10 chart, and put the
combined chart side-by-side with the User Top 10 ranking in one row
with the same 16/8 column ratio used by the System Load section above.
* New component usage-charts/usage-by-model.tsx owns both metric queries
(group_by=['date','route']), aggregates a shared route→color map so
the legend toggles both donuts together, and renders two pies at
centers ('20%','50%') and ('60%','50%') with radius ['50%','70%'].
* new-usage.tsx no longer mounts ApiRequestsByModel / TokenUsageByModel
/ TopTokenUsageByApiKey; layout collapses to two columns sized
(lg=24 xl=16) and (lg=24 xl=8).
* Delete unused token-usage-by-model.tsx, api-requests-by-model.tsx,
top-token-usage-by-api-key.tsx, and the use-top-token-usage-by-api-key
hook.
* Strip the now-unreferenced dashboard.tokenUsageByModel /
apiRequestsByModel / topTokenUsageByApiKey locale keys across 5
locales, add the new dashboard.usageByModel title; locale parity
check passes.
