fix(api): parse HTTP-date in Retry-After and eliminate unreachable rate-limit fallback (#572)

[Proxima84-code]

Summary

Fixes #572.

In apiRequest 429 response handling:

1. RFC 9110 HTTP-date support: parseInt was failing when Retry-After contained standard HTTP-date timestamps (e.g. Retry-After: Wed, 21 Oct 2026 07:28:00 GMT), evaluating to NaN and losing retry throttle delays.
2. Dead code removal: The template literal on the left side of || always evaluated to truthy, rendering the fallback message Rate limited. Please try again later. unreachable.

Changes

• Added and exported parseRetryAfter in src/lib/api.ts handling delay-seconds (/^\d+$/) and HTTP-dates via Date.parse.
• Structured error message selection to explicitly use countdown formatting when seconds are defined, or generic rate-limit text otherwise.
• Added comprehensive unit tests in src/lib/api.test.ts verifying delay-seconds, HTTP-dates, missing headers, and malformed strings.

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