fix(overview): pin number grouping to en-US so output is locale-indep…

[NihalJain]

…endent

codeburn overview formatted token/call/session counts with a bare toLocaleString(), which groups digits according to the host's locale. On an Indian-locale machine (en-IN) a value like 2,002,000,000 renders as 2,00,20,00,000, so the rendered summary — and its snapshot test — differed by machine. (formatCost already avoided this via an explicit thousands regex.)

Pin formatTokens to 'en-US' and route the remaining count columns through a new formatCount helper that does the same, so the overview output is byte-identical everywhere.

Verify: npm test -- overview (previously failed on non-US locales; the "thousands separators" case now passes regardless of $LANG / ICU default).

Summary

Testing

• I have tested this locally against real data (not just unit tests)
• npm test passes
• npm run build succeeds

For new providers only:

• I installed the tool and generated real sessions by using it
• npm run dev -- today shows correct costs and session counts for this provider
• npm run dev -- models --provider <name> shows correct model names and pricing
• Screenshot or terminal output attached below proving it works with real data

[iamtoruk]

Verified: reproduced the failure on main under an en-IN locale (grouping renders 2,00,20,00,000), confirmed this branch fixes it, and re-ran the overview tests after merging current main into the branch locally. All call sites in overview.ts are covered. The same bare toLocaleString() pattern exists in six other files; that sweep is follow-up material, not this PR's scope.