docs: add an animated LNURL-pay flow diagram to the project page

[Chemaclass]

Summary

Adds two graphics to the project page so a reader can see how a Lightning Address works
instead of parsing four paragraphs about it.

Address anatomy

A static figure that makes the central point: a Lightning Address is not new protocol.
bob@example.com splits into the domain you control and a well-known path, which join
into the URL a wallet opens — the URL this library answers.

Payment flow

A seven-step sequence diagram between the payer's wallet, your domain (highlighted as
"this library") and your node:

1. GET /.well-known/lnurlp/bob
2. pay params — min, max, callback
3. GET /bob?amount=2000
4. POST /api/v1/payments to your backend
5. bolt11 invoice back
6. { "pr": "lnbc20n1p…" } to the wallet
7. the wallet pays the invoice over Lightning

A packet travels along the active arrow, the step is described underneath in plain
language, and the surrounding copy calls out which steps the library answers and that the
sats never pass through it.

Behaviour

• Autoplays only while the diagram is in the viewport (IntersectionObserver), stops when
  the tab is hidden.
• ‹ / Pause / › plus numbered step buttons; any manual interaction stops autoplay for
  good.
• Under prefers-reduced-motion: reduce autoplay is off and the packet does not animate —
  the diagram still works as a manual stepper.
• No external assets: inline SVG, inline CSS, ~90 lines of vanilla JS. Works with the same
  light/dark handling as the rest of the page.

Verification

Rendered headless (Chrome) in both colour schemes and at narrow widths:

• light and dark palettes both check out, including the teal/amber pairing in the figures
• no horizontal overflow: scrollWidth == clientWidth, no offending elements
• inline <code> now wraps (overflow-wrap: anywhere) so long URLs cannot push the page
  sideways on a phone

Also adds the CHANGELOG entry for the Pages site, which #38 did not include.

Test plan

• HTML well-formed, script passes node --check, 7 captions ↔ 7 diagram steps
• composer test-all unaffected and green
• Pages deployment renders the new section after merge