fix(www): prevent build failure when NEXT_PUBLIC_APP_URL is missing

[Yeom-JinHo]

Description

This PR focuses on fixing production build failures when NEXT_PUBLIC_APP_URL is missing in the www app.

Changes

• URL Fallback (major fix): Updated absoluteUrl() to use siteConfig.url when NEXT_PUBLIC_APP_URL is not set.
• Metadata Generation: Replaced direct environment-based OG URL creation with absoluteUrl("/og") in docs and showcase metadata.
• URL Safety: Switched to new URL()-based composition to ensure valid absolute URLs for metadata and structured data.
• Type Cleanup: Changed Metadata import to import type in utils.ts.

Motivation

• Production deployments were failing during SSG/prerender with TypeError: Invalid URL when NEXT_PUBLIC_APP_URL was empty or undefined.
• Metadata code in docs/showcase relied on direct env string interpolation, which made builds fragile across environments.
• This change makes build behavior deterministic and environment-agnostic by using a stable fallback base URL.

Breaking Changes

• None.
• Existing behavior is preserved when NEXT_PUBLIC_APP_URL is provided.
• When missing, the app now safely falls back to siteConfig.url instead of failing the build.

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