Add HTTP error status validation to API fetch calls

[Copilot]

All fetch calls were calling .json() without checking response.ok, causing unhandled promise rejections on non-200 status codes.

Changes

• Added response.ok validation before parsing JSON for all four API endpoints:

  • /api/usage
  • /api/costs/summary
  • /api/models/pricing
  • /api/recommendations

• Errors now include HTTP status code and status text for debugging

Example

const usageRes = await fetch(`${API_BASE_URL}/api/usage?limit=100`);
if (!usageRes.ok) {
  throw new Error(`Failed to fetch usage data: ${usageRes.status} ${usageRes.statusText}`);
}
const usageData = await usageRes.json();

Errors are caught by existing try-catch and displayed to users via the error banner component.

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