fix(api): parse Retry-After HTTP-date format and remove dead fallback code (#572)

[PINYOPATTANAWASANPORN]

Summary of Changes

Fixed 429 rate-limit handling in apiRequest by supporting RFC 9110 HTTP-date strings in Retry-After headers and removing dead fallback code (#572).

Root Cause / Technical Context

1. apiRequest previously used parseInt(retryAfter, 10), which returned NaN when Retry-After was formatted as an HTTP-date string (e.g. Sun, 04 Oct 2026 21:40:00 GMT), causing the retry-after duration to be ignored.
2. The template string `You're doing that too fast.${waitMsg}` is non-empty, rendering the right operand of || "Rate limited. Please try again later." completely unreachable dead code.
3. Fix details:
   • Added parseRetryAfter helper supporting integer seconds and RFC 9110 HTTP-date headers.
   • Removed dead fallback OR expression.
   • Added unit test suite in api.test.ts validating integer seconds, HTTP-date parsing, and 429 error formatting.

Verification & Testing

Ran npx jest src/lib/api.test.ts --coverage=false:

PASS src/lib/api.test.ts
  fetchIndexableReputationHandles
    √ returns only profiles that explicitly opted in (4 ms)
    √ returns nothing when the backend omits the flag entirely (1 ms)
    √ drops entries with a blank username
    √ falls back to the caller's list when the backend is unreachable
  parseRetryAfter
    √ parses integer seconds correctly (2 ms)
    √ parses HTTP-date headers into remaining seconds (1 ms)
  apiRequest rate limiting (429)
    √ formats 429 error message correctly with integer Retry-After (8 ms)
    √ formats 429 error message correctly without Retry-After header

Test Suites: 1 passed, 1 total
Tests:       8 passed, 8 total
Snapshots:   0 total
Time:        1.389 s

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