Fix blog index chat opener

[ZaynJarvis]

Summary

• replace the index-page circular Zouk launcher with the same right-edge line toggle used by post pages
• keep the post-page bottom composer behavior unchanged
• keep an index-page edge toggle visible on mobile where there is no bottom composer fallback

Verification

• npm run build
• npx playwright screenshot --browser=chromium --viewport-size=1280,900 http://127.0.0.1:5175/?lang=en /tmp/ov-blog-index.png
• npx playwright screenshot --browser=chromium --viewport-size=1280,900 http://127.0.0.1:5175/post/openviking-context-database-architecture/?lang=en /tmp/ov-blog-post.png
• npx playwright screenshot --browser=chromium --viewport-size=390,844 http://127.0.0.1:5175/?lang=en /tmp/ov-blog-index-mobile.png
• npx playwright screenshot --browser=chromium --viewport-size=390,844 http://127.0.0.1:5175/post/openviking-context-database-architecture/?lang=en /tmp/ov-blog-post-mobile.png

[github-actions]

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🧪 No relevant tests

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