Add listing detail page

[strongkeep-debug]

This PR adds a standalone listing detail page for the x402 Directory. Each listing exposes name, URL, description, category, pricing, verification status, last checked date, maintainer metadata, and a direct try link.

Shipyard bounty: bnt_pqhpsacw — Add listing detail page.

Area	Change	Evidence
Listing detail	Renders details for each sample x402 listing with direct ?listing= links	src/main.js, src/listings.js
Pricing/status helpers	Formats endpoint pricing and verification labels	tests/listing-detail.test.mjs
Visual surface	Adds responsive UI and custom thumbnail	src/styles.css, public/thumbnail.svg

Validation run locally:

npm test
# 3 pass, 0 fail

git diff --check
# exit 0; no whitespace errors (Windows line-ending warning only)

static smoke check
# index/main.js/thumbnail returned 200; detail script includes endpoint URL, pricing, verification, last checked, and try-link sections