Warn before banked reset credits expire

[Tylerjet]

Problem

T3 Code now shows banked Codex reset credits and provides a confirmed Use a reset credit action, but users are not proactively warned before a banked credit expires. The expiry can therefore be visible in Usage → Limits and still be easy to miss.

This follows the optional-warning part of #5539 and builds on the reset-credit support added in #9534.

Proposed behavior

• Warn during the final seven days before the soonest banked reset credit expires.
• Show the reminder at most once per distinct expiring-credit set per app launch.
• Use a persistent warning toast on web/desktop and a native alert on mobile.
• Make View limits the only reminder action and open Usage → Limits directly.
• Highlight the expiry countdown with the existing semantic warning color.
• Deduplicate the same signed-in provider account across connected environments, using its earliest reported expiry.

Safety

The reminder must never redeem or claim a credit. The existing Use a reset credit flow remains separate and continues to require its confirmation dialog.

I have a tested implementation ready for web/desktop and mobile.