Align OpenAI Codex usage stats with reset windows

[paraliine]

Summary

This PR aligns OpenAI OAuth Codex usage window stats with the displayed reset windows.

Previously, the 5h and 7d local stats were always queried using rolling windows:

• now - 5h
• now - 7d

That could make requests/tokens/cost differ from the actual quota window shown by the Codex utilization and
reset time.

Changes

• Added a small helper to derive the stats start time from the already parsed UsageProgress.ResetsAt.
• For active reset windows, stats now use:
  • 5h: resets_at - 5h
  • 7d: resets_at - 7d
• If the reset time is missing or expired, behavior falls back to the previous rolling window logic.

Notes

This keeps the change minimal and reuses the existing parsed UsageProgress data instead of reparsing raw
accounts.extra fields.

Closes #2954

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