[WEB-4406] - Fix Insulin Averaging due to Partial Days

[henry-tp]

WEB-4406

For further context: https://tidepoolteam.slack.com/archives/C09AWCB4Y/p1770862343039929

Blip: tidepool-org/blip#1872

[Copilot]

Pull request overview

This PR fixes a bug in the insulin and carbohydrate averaging calculations that were previously counting calendar days instead of actual 24-hour periods from the first datum. This caused inaccurate averages when data was collected at times that didn't align with calendar day boundaries (e.g., starting at 8pm instead of midnight).

Changes:

• Replaced calendar day counting with 24-hour bucket calculation based on elapsed time from the first datum
• Applied the same fix to both insulin data (getInsulinData()) and carbohydrate data (getCarbsData()) methods
• Added comprehensive test coverage for partial-day and multi-day scenarios

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/utils/StatUtil.js	Replaced calendar day counting logic with 24-hour bucket calculation in getInsulinData() and getCarbsData() methods; removed unused formatLocalizedFromUTC import
test/utils/StatUtil.test.js	Added comprehensive test cases for both insulin and carbs data covering partial-day and multi-day scenarios
package.json	Updated version from 1.55.0-web-4384-date-mismatch.5 to 1.55.0-web-4406-insulin-calc.1

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