feat(07-02): add resolveOgImage helper + og-blog-default.jpg fallback asset

- app/utils/resolve-og-image.ts: absolutises frontmatter image or falls back to /og-blog-default.jpg
- public/og-blog-default.jpg: placeholder (copied from og-image.png) — branded 1200x630 design follow-up pending

app/utils/resolve-og-image.ts

@@ -0,0 +1,14 @@
+/**
+ * Resolves an article's og:image to an absolute URL.
+ * Strategy (D-05): frontmatter `image` if present, else branded fallback `/og-blog-default.jpg`.
+ * Consumed by: app/pages/blog/[slug].vue (useSeoMeta.ogImage + defineArticle.image)
+ *              app/pages/blog/index.vue (useSeoMeta.ogImage fallback only).
+ */
+const SITE_URL = 'https://killiandalcin.fr'
+const FALLBACK = '/og-blog-default.jpg'
+
+export function resolveOgImage(article?: { image?: string } | null): string {
+  const raw = article?.image?.trim() || FALLBACK
+  if (raw.startsWith('http://') || raw.startsWith('https://')) return raw
+  return `${SITE_URL}${raw.startsWith('/') ? raw : `/${raw}`}`
+}