chore(icons): refactor post-icon mixin to dynamicallly load icons as css files

[alionazherdetska]

📄 Description

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📝 Checklist

• ✅ My code follows the style guidelines of this project
• 🛠️ I have performed a self-review of my own code
• 📄 I have made corresponding changes to the documentation
• ⚠️ My changes generate no new warnings or errors
• 🧪 I have added tests that prove my fix is effective or that my feature works
• ✔️ New and existing unit tests pass locally with my changes

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[sonarqubecloud]

Quality Gate passed

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[swisspost-bot]

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