[plugins] Pad selected plugin logos

[ssetty-oai]

Summary

• Add 192x192 padded icon/logo assets for the plugin icons called out by openai/openai#996735 that are owned in this repo: Base44, fal, Figma, OpenAI Developers, Picsart, Shutterstock, Shopify, Supabase, and Vercel.
• Point the changed plugins' logo fields at the padded assets, and point remaining tight composerIcon fields at the same padded assets where present.
• Keep Vercel on a fully white 192x192 canvas so its white app-icon background is extended consistently.
• Leave Browser, Databricks Genie, Sites, and Snowflake untouched because matching plugin directories are not present in this repository.

Validation

• jq empty on the changed plugin manifests
• git diff --check
• Targeted script verifying generated PNG outputs and manifest icon paths
• Rendered Shopify and Supabase SVG outputs locally with rsvg-convert for visual inspection

Related client-side workaround: https://github.com/openai/openai/pull/996735