Import preact side effect for setting up signals binding

[vividviolet]

Background

This PR adds the missing import for @shopify/ui-extensions/preact across multiple extension templates to ensure proper functionality of Preact-based UI extensions.

Solution

Added the import statement import '@shopify/ui-extensions/preact'; to all Preact-flavored extension templates. This import is necessary to properly initialize the Preact environment for Shopify UI extensions and ensure all required dependencies are available at runtime.

Checklist

• I have 🎩'd these changes
• I have squashed my commits into chunks of work with meaningful commit messages

[vividviolet]

• Fix remote ui template paths #262
• Import preact side effect for setting up signals binding #261 👈 (View in Graphite)
• main

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