fix(usage): Usage tab displays dates in UTC instead of user's local timezone

[IamCoder18]

Summary

The usage tab displays usage data grouped by UTC dates instead of the user's local timezone. This affects both "By Day" and "By Model & Day" views, as well as the streak calculation and streak calendar.

Affected Components

• Usage tab "By Day" view
• Usage tab "By Model & Day" view
• Streak calculation
• Streak calendar

Problem

When viewing usage data, dates are grouped by UTC day rather than the user's local day. For example, a user in PST (UTC-8) coding at 8 PM will have their usage recorded as occurring on the next day in UTC.

Possible root cause: The API route /api/profile/usage/route.ts uses DATE(created_at) which extracts the date in the database's timezone (UTC), and the streak calculation uses JavaScript's toISOString() which always returns UTC.

Steps to Reproduce

1. Set browser timezone to a non-UTC timezone (e.g., PST, EST)
2. Navigate to the usage tab
3. View "By Day" or "By Model & Day" view
4. Observe that dates shown don't match local date boundaries

Expected Behavior

• Usage data should be grouped by the user's local date
• Streak calculation should count consecutive days in the user's local timezone
• Streak calendar should display dates in the user's local timezone

Actual Behavior

• Usage data is grouped by UTC date
• Streak calculation uses UTC-based date comparison
• Dates displayed don't match user's local date boundaries

Possible Root Causes

• API route uses DATE(created_at) PostgreSQL function without timezone conversion
• Client-side streak calculation uses toISOString().split('T')[0] which is UTC-based
• Browser timezone is not detected or passed to the API

Environment

• Component: Usage tab (/app/(app)/usage/page.tsx)
• API: /api/profile/usage/route.ts
• Database: PostgreSQL with microdollar_usage table