Fix pool performance return calculation

[JeremyZeng77]

Fixes #6

Summary

• limit pool performance deposits/withdrawals to the selected weekly/monthly/yearly stats window
• calculate period income as ending balance plus withdrawals minus starting balance minus deposits
• use the midpoint capital estimate discussed in the issue thread as the return denominator, instead of ending balance
• guard zero/negative estimated capital to avoid nonsensical percentages

Verification

• git diff --check
• npm run build

Bounty payout

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[JeremyZeng77]

Local verification passed with git diff --check and npm run build.

The only failing GitHub status I see is Vercel with Account is blocked., which appears to be an external project/account deployment issue rather than a code failure in this branch.