refactor: convert usePluginsStore to Composition API

[elevatebart]

And fix loading of icons

[Copilot]

Pull Request Overview

This PR converts the plugins store from Pinia's Options API to the Composition API format and fixes icon loading functionality. The refactor maintains the same external interface while modernizing the internal implementation.

Key changes:

• Converted from Options API (state/getters/actions) to Composition API (ref/computed/functions)
• Split icon management into separate apiIcons and pluginsIcons refs with a computed icons property
• Replaced this.$http with the useAxios composable for HTTP requests

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[github-actions]

🐋 Docker image

ghcr.io/kestra-io/kestra-pr:11965

docker run --pull=always --rm -it -p 8080:8080 --user=root -v /var/run/docker.sock:/var/run/docker.sock -v /tmp:/tmp ghcr.io/kestra-io/kestra-pr:11965 server local

[Piyush-r-bhaskar]

looks good