Fix/usage timezone handling

[IamCoder18]

Summary

Fix timezone handling in the usage tab so dates are grouped by the user's local timezone instead of UTC. This affects "By Day", "By Model & Day" views, streak calculation, and streak calendar.

Changes:

• API route /api/profile/usage now accepts timeZone parameter and uses PostgreSQL AT TIME ZONE for timezone-aware date grouping
• Client detects browser timezone via Intl.DateTimeFormat().resolvedOptions().timeZone
• Streak calculation and calendar data generation now use toLocaleDateString('en-CA', { timeZone }) for correct local date formatting
• Initial timezone state is null to defer query until browser timezone is detected

Visual Changes

N/A - functionality fix, dates now correctly reflect user's local timezone.

Reviewer Notes

• The en-CA locale is used because it formats dates as YYYY-MM-DD by default
• Added isValidTimezone() function to validate IANA timezone strings and prevent SQL injection
• API falls back to UTC if invalid or missing timezone parameter

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (2 files)

• apps/web/src/app/(app)/usage/page.tsx
• apps/web/src/app/api/profile/usage/route.ts

---

_{Reviewed by gpt-5.5-20260423 · 310,160 tokens}