fix(ui): flip /usage gauge color thresholds

[he-yufeng]

Fixes #2019.

_format_row in src/kimi_cli/ui/shell/usage.py computes

ratio = (row.limit - row.used) / row.limit if row.limit > 0 else 0

i.e. the remaining fraction (and the caller renders it as {percent:.0f}% left). The color helper below reads ratio as if it were a "used" gauge:

def _ratio_color(ratio: float) -> str:
    if ratio >= 0.9:
        return "red"      # actually: 90%+ remaining, should be green
    if ratio >= 0.7:
        return "yellow"   # 70%+ remaining, should be green too
    return "green"        # < 70% remaining, gauge silently goes green as it drains

So the progress bar is green while usage is almost full and turns red right after a reset. This PR flips the thresholds so red kicks in when ≤10% remains and yellow when ≤30% remains, which matches the % left label.

Reporter (@sunmy2019) also gave the same fix in the issue. Added a short comment explaining the direction since the variable name alone doesn't make it obvious.

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[devin-ai-integration]

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