fix(stats): align timezone handling with user preferences and UTC params

[billyvg]

Fixes two bugs on the Stats page (/settings/stats/) where the chart date range didn't match the time selector and the timezone label ignored user account preferences.

What changed

index.tsx — dataDatetime getter: Changed moment(start).utc() to moment.utc(start). The prior form treated the raw URL start/end params (which have no timezone suffix) as local time before converting to UTC, adding an extra offset. DatePageFilter treats the same strings as already-UTC, so the chart was systematically shifted by the user's UTC offset relative to the time selector.

usageStatsOrg.tsx — chartDateRange memo: Replaced .local() with .tz(getUserTimezone()) for displayStart/displayEnd. moment().local() resolves to the browser system timezone, not the user's Sentry account timezone preference. This caused the chart date display and the Date Range: footer label to show the wrong timezone when user preferences differed from the browser.

usageChart/utils.tsx — getDateFromMoment: Same .local() → .tz(getUserTimezone()) fix for the x-axis date label generation.

Follows the same pattern already used by timeSeriesWidgetVisualization.tsx and heatMapWidgetVisualization.tsx.

What's verified

• 3-file diff is minimal and surgical; no logic changes beyond the timezone resolution
• getUserTimezone() is a stable export from sentry/utils/dates already imported throughout the codebase
• Existing tests for usageChart/utils.spec.tsx and index.spec.tsx cover the datetime logic

Fixes #119327

--

View Junior Session in Sentry

[billyvg]

Tested this on preview: load stats page, observe timestamps in date picker, chart tooltips, and the date range label below chart. change account tz in different tab. refresh dev preview and observe new timestamps.