Toolbar shows false '100% extra usage' alert on session start

Bug description

On session start, the toolbar briefly shows a false alert:

You've used 100% of your extra usage · resets [date]

The alert is inaccurate — web dashboard shows correct (non-100%) usage. It disappears after the first real user message.

Root cause (from binary analysis)

Claude Code v2.1.104 runs a startup quota probe during "Starting background startup prefetches":

1. mG9() calls TY1() — a real API call with {role:"user", content:"quota", max_tokens:1, source:"quota_check"}
2. Response headers contain anthropic-ratelimit-unified-overage-utilization = 1.0
3. FU6(headers) processes these → sets ok.utilization = 1.0, ok.rateLimitType = "overage"
4. Toolbar renders the 100% alert immediately
5. First real user message → fresh API call → headers reflect actual state → alert clears

The probe appears to return stale/cached rate limit headers that don't match the actual usage state returned by subsequent real API calls.

Steps to reproduce

1. Use Claude Code until extra usage is near or at 100%
2. Close the session
3. Top up or wait for state to normalize (web shows non-100%)
4. Open a new Claude Code session
5. Toolbar shows 100% alert immediately
6. Send any message → alert disappears

Expected behavior

Toolbar should either:

• Not show the alert until a real message confirms the rate limit state, or
• Show a "checking..." state while the startup probe is in-flight

Environment

• Claude Code: 2.1.104
• OS: macOS Darwin 25.3.0 arm64

Suggested fix

Debounce or suppress the overage alert until at least one real user message confirms the high utilization state. The startup probe result alone should not trigger the display.