Centralize PROJECT_* in feeds and sitemap

Sveleton is a bootstrap; re-using it for a new project should mean changing PROJECT_NAME, PROJECT_URL, and PROJECT_DESCRIPTION in one place. Today, several SEO outputs bypass these constants.

Current State

• feed.json/+server.js:7,17 — hardcodes 'Sveleton' and 'Latest articles from Sveleton'
• feed.json/+server.js:15 — home_page_url uses empty base instead of PROJECT_URL
• feed.xml/+server.js:23,43 — empty BASE_URL; <author><name>Team</name> placeholder
• sitemap-appendix.xml/+server.js:5 — hardcoded 'https://sveleton.ramilkarimov.me'

Tasks

• Replace hardcoded strings with imports from $lib (PROJECT_NAME, PROJECT_URL, PROJECT_DESCRIPTION)
• Remove the placeholder <author><name>Team</name>
• Fix the trailing // in feed.xml <id> (feed.xml/+server.js:40)