[0 BOUNTY] [TypeScript] Harden API response error handling

[alKanz17]

What this does

Prevents 4xx/5xx HTTP responses from flowing through response interceptors as successful data. Non-OK responses are now normalized into ApiError objects and thrown through the error interceptor chain.

How it works

• parseResponse() now throws an ApiError for !response.ok so callers no longer need to duplicate status checks
• Added parseErrorResponse() that preserves request IDs, status codes, structured JSON error payloads from the backend, and actionable suggestions
• Added getSuggestionForStatus() with human-readable messages for 401, 403, 404, 429, and 5xx errors
• Added isApiError() type guard so downstream code can distinguish ApiErrors from network/abort errors
• Error interceptors (401 token refresh, 429 rate limiting) are now reliably exercised on every error response

Testing

• tsc -b passes with zero errors
• vite build succeeds
• The existing error interceptor for 401 (token refresh) and 429 (backoff) now fires correctly because non-OK responses are thrown instead of returned as ApiResponse

Closes #1