Improve error message when account has no API credits (currently shows confusing "Rate limit reached")

Description

When using Claude Code with Opus 4.6 (1M context window) and switching between accounts, if the destination account has no API credits, the error message displayed is simply "Rate limit reached." This is misleading and confusing.

Problem

• The error "Rate limit reached" suggests the user has hit a usage cap or that Claude Max is trying to authenticate via an API key
• The actual issue is that the account lacks API credits, and this is easy to miss — especially when using the 1M context window, which requires API credits
• Users who pay for Claude Max (e.g., 20X plan) may not realize that the 1M context window specifically needs API credits on the active account
• This is especially confusing when switching between personal and business accounts, where one account may have credits and the other doesn't

Expected Behavior

The error message should:

1. Clearly indicate that the account lacks API credits for the 1M context window
2. Suggest the user add credits to their account (with a link if possible)
3. Differentiate between "rate limit hit" (temporary, wait and retry) vs "no credits available" (action required: add credits)

Steps to Reproduce

1. Have a Claude Max subscription on one account
2. Switch to another account (e.g., business email) that has no API credits
3. Try to use Claude Code with Opus 4.6 (1M context)
4. Observe the confusing "Rate limit reached" error

Environment

• Claude Code CLI
• Model: Opus 4.6 (1M context window)