fix: permanent [Timeout] in extra usage widgets when no monthly limit is set

[mhriemers]

Summary

Fixes #413 — accounts with extra usage enabled but no monthly limit configured (monthly_limit: null) saw the extra-usage widgets render a permanent [Timeout] even though every API fetch succeeded.

Root cause

hasRequiredUsageField() treated the absence of extraUsageLimit/extraUsageUtilization as incomplete data. For limit-less accounts those fields never appear in the API response, so:

1. cached data (memory + file) was never accepted for widgets requiring them,
2. the fetch loop re-fetched every cycle, and
3. getStaleUsageOrError() surfaced the lock error as [Timeout] — indefinitely, since no amount of refetching can produce a field the account doesn't have.

Fix

Extend the existing extraUsageEnabled === false special case to any response where the extra usage state is known: once the API has reported is_enabled, absent detail fields are conclusive.

With this change the widgets render nothing for limit-less accounts (instead of a false network error), matching how other widgets handle genuinely unavailable data. #414 proposes showing used_credits for these accounts.

Testing

• New regression test treats enabled extra usage without a monthly limit as complete for extra usage widget fields, with a fixture mirroring a real monthly_limit: null API response — fails without the fix (cache rejected, second probe re-fetches), passes with it
• Full suite: 1394 tests pass, bun run lint clean

🤖 Generated with Claude Code

[mhriemers]

Pushed a small test tweak: removed currency: 'EUR' from the new fixture so this PR and #418 stay green regardless of merge order (#418 starts parsing that field, which would have added extraUsageCurrency to the parsed result and broken this test's expectation).