Improve SEO metadata consistency and add structured-data fallbacks

[AvdLee]

Motivation

• Fix inconsistent canonical and image URLs used across pages by enforcing absolute, production-safe metadata URLs and consistent trailing-slash behavior based on site config.
• Improve social previews and Twitter/Open Graph metadata coverage with centralized fallbacks so pages without explicit overrides still produce valid previews.
• Make JSON-LD output more consistent and lightweight by ensuring Organization schema is always present and avoiding noisy runtime date generation for non-article pages.
• Keep changes incremental and low-risk by reusing existing SEO/Head/StructuredData components without changing rendering or build pipelines.

Description

• Added a small reusable helper src/lib/utils/seo.ts exposing toAbsoluteUrl and normalizeCanonicalUrl to produce absolute URLs and normalized canonical links.
• Hardened src/layouts/components/SEO.astro to use normalizeCanonicalUrl for canonical normalization, and a robust OG image fallback chain (page override → site meta image → app icon) and switched Twitter tag attributes to name= for consistency.
• Extended src/layouts/components/StructuredData.astro to always emit a lightweight Organization schema and made WebPage date fields optional while removing runtime new Date() for non-article page types.
• Updated docs Starlight head src/components/starlight/Head.astro to add og:url, a canonical link, site-wide OG/Twitter image fallbacks, and an SSR-compatible BreadcrumbList JSON-LD for /docs/* routes derived from path segments.

Testing

• Ran lint: cd docs && npm run lint — passed.
• Ran format check: cd docs && npm run format:check — passed (Prettier auto-fixed then rechecked).
• Ran typecheck: cd docs && npm run typecheck — passed (one existing TS hint unrelated to these changes).
• Ran build: cd docs && npm run build — failed in this environment due to an external WordPress API fetch in src/pages/blog/[slug].astro returning ENETUNREACH, not caused by the metadata changes.

---

Codex Task