Wire cover images into the five new tutorial articles

[ethanwestfall]

Summary

• PR Add cover images to five new articles #31 added SVG sources and rendered PNGs for five new tutorial articles (codex-multi-model-subagents, configure-claude-code-app-with-cc-switch-and-token-station, configure-claude-code-cli-with-token-station, configure-codex-app-with-token-station, configure-codex-cli-with-token-station), but never added the cover frontmatter field to any of them
• Confirmed via the /blog/ listing page: all five still showed generic gradient placeholders instead of their (already-committed) cover art
• Verified all 20 article files (5 articles × 4 languages) were missing cover: entirely, and that each corresponding PNG already exists in blog/ and matches its article's content
• Adds cover: "blog/<slug>-cover.png" to all 20 files, same filename shared across all four language versions per the site's convention
• No new image generation needed; this is purely wiring already-existing, already-approved cover art into the frontmatter

Test plan

• npm run build succeeds
• /blog/ listing shows real cover art (not gradient placeholders) for all five articles
• og:image is present and correct on each of the 20 built pages