fix(www): use siteConfig.url in sitemap and restore static generation

[zigzagdev]

Description

Showcase pages were never listed in sitemap.ts, leaving them out of search indexing entirely. Also switches domain resolution from the request Host header to siteConfig.url for consistency with the rest of the app, which lets the route go back to static generation.

Changes

• Add showcase pages (showcaseSource) to the generated sitemap
• Replace headers()-based host resolution with siteConfig.url, matching every other page in the app (robots.ts, absoluteUrl, etc.)
• Re-enable static generation (revalidate = false, dynamic = "force-static") for app/sitemap.ts, since it no longer depends on a dynamic API

Motivation

/showcase/[...slug] pages are real, crawlable pages with their own generateStaticParams, but were silently excluded from the sitemap — hurting their discoverability in search. The route also trusted the request Host header to build canonical URLs, which is inconsistent with the rest of the app and forced the route off static generation on every request.

Breaking Changes

None.

Screenshots

Not applicable — no UI change.

Checklist

• pnpm check passes locally — not run in this environment (dependencies not installed); please verify in CI
• pnpm build passes locally — not run in this environment (dependencies not installed); please verify in CI
• pnpm build:registry was run and the generated files are committed — not applicable, no registry/ or config/site.ts changes
• PR title follows Conventional Commits

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[Yeom-JinHo]

@zigzagdev Thanks for the PR! I pushed a small follow-up to your branch that drops the showcase entries from the sitemap.
Those detail pages are pretty thin (just a title and a card), so I'd rather not submit them for indexing. The siteConfig.url and static generation changes are great, keeping those as-is.