feat: handle GitHub API rate limits with user-visible feedback (#93)

[Harsh-Codes-77]

Handle GitHub API rate limits with user-visible feedback

Implements graceful rate limit handling for GitHub API 429 responses:

• New githubFetch wrapper utility centralizes 429 detection and throws typed RateLimitError
• New RateLimitBanner component displays rate limit status with countdown to reset
• Updated 4 API metrics routes to catch rate limits and return 429 responses with reset timestamp
• Updated 4 dashboard components to detect rate limits and show banner instead of generic error

Users now get immediate, friendly feedback when hitting GitHub API limits instead of confusing error messages.

[vercel]

@Harsh-Codes-77 is attempting to deploy a commit to the PRIYANSHU DOSHI's projects Team on Vercel.

A member of the Team first needs to authorize it.

[Priyanshu-byte-coder]

1. Hardcoded Tailwind colors in RateLimitBanner.tsx — border-amber-500/25, bg-amber-50, text-amber-700 don't respect the theming system. Use border-[var(--warning)]/25 or var(--destructive) variants, or define a --warning CSS var.

2. Missing EOF newlines on githubFetch.ts and RateLimitBanner.tsx.

3. 403 + X-RateLimit-Remaining: 0 not handled — GitHub often returns 403 before ever sending 429 when limits are exhausted. Add detection: if response.status === 403 && response.headers.get('X-RateLimit-Remaining') === '0', treat as rate-limited.