Add per-section clap button

[GrzywN]

Adds a clapping feature to the guide — readers can express appreciation for individual sections, not just whole articles.

What's included

ClapButton Astro component (src/components/ClapButton.astro)

• Fetches current clap count on page load via GET /claps?slug=
• Accumulates clicks locally, sends them in a single sendBeacon on page leave (pagehide)
• Shows a conic-gradient progress ring reflecting how many claps the user has given (across all sessions, via user_claps from the API)
• Graceful degradation: if PUBLIC_API_URL is not set, renders silently without errors
• Runtime validation of the API response via Zod (astro/zod)

Technical decisions worth noting:

• State split into local (session claps, sent flag) and remote (server response) for clarity
• remote.count++ on click is an intentional optimistic update — keeps displayed count in sync without a round-trip

Placed after every ## section across all 17 articles (60 sections total), each with a unique slug matching the article path + section heading.

.env.example updated with PUBLIC_API_URL.

API

The component talks to a separate self-hosted API (agentic-engineering-applause-feature repo). Set PUBLIC_API_URL in .env to point at it.

GET /claps?slug={slug} → { count, max_claps, user_claps }
POST /claps?slug={slug} body: { claps: number }