[bug] usage hard-fails on Starter plan (no AI-cost box) and localized labels

[terje1965]

What happened?

kagi usage fails hard on plans that don't include the AI-cost box (reproduced on Starter):

$ kagi usage
parse error: Kagi billing page did not contain the current AI cost and limit; the page layout may have changed

Root cause

parse_usage_html in src/usage.rs requires the label total ai cost this period (or ai usage (usd)) and returns a parse error when absent. But the AI-cost box is an Ultimate-plan feature. A Starter billing page (verified against /settings/billing server HTML, 2026-09-12, Norwegian locale) instead renders plain usage counters:

<span>Starter</span> …
<div class="billing_box_count_title">Søk</div>
<div class="billing_box_count_num">…3…/300</div>
<div class="billing_box_count_title">Assistant-interaksjoner</div>
<div class="billing_box_count_num"><span >8</span>/300</div>
<div class="billing_box_count_title">Saldo</div>
<div class="billing_box_count_num">$0.07</div>
<span>Neste fornyelse er <b>2026-09-19</b></span>

Two separate issues:

1. ai_cost is treated as mandatory, so every non-Ultimate account gets a hard parse error even though plan/balance/renewal data is present.
2. Labels are matched in English only. account balance / next renewal is never match localized pages (Saldo, Neste fornyelse er), so those fields come back empty on non-English accounts even when present.

Suggested fix

• Make ai_cost: Option<AiCostUsage> and skip the field when the box is absent (JSON consumers already tolerate skip_serializing_if-style fields).
• Accept localized variants for the labels that have stable equivalents (or, better long-term, parse the billing_box_count_* structure instead of label text).

I have implemented this locally (optional ai_cost + saldo/neste fornyelse er fallbacks) and verified E2E on a Norwegian-locale Starter account:

{
  "plan": "Starter",
  "ai_cost": null,
  "account_balance_usd": 0.07,
  "next_renewal": "2026-09-19",
  "daily_usage": []
}

All 215 unit tests pass (one test updated: the "missing cost pair errors" test now asserts successful parse with ai_cost: None). Happy to open a PR.

Environment

• kagi-cli 0.20.0 (homebrew tap microck/kagi)
• macOS 15 (arm64)
• Kagi plan: Starter; UI language: Norwegian (Bokmål)