feat(themes): channel visibility for home blocks

[eslamoo]

What

Adds channel visibility to theme-raed home blocks. A merchant can now show a component on Desktop only, Mobile web only, or both, without writing custom CSS.

Jira: https://salla-dev.atlassian.net/browse/TD-13969

How

• New s-hide-on-mobile / s-hide-on-desktop utility pair in src/assets/styles/05-utilities/channel-visibility.scss, breaking at 768px (Tailwind md).
• Every home block root <section> derives its class inline from component.channel_visibility: { mobile, desktop }, which the backend injects.

Notes

• Plain CSS rather than Tailwind md:hidden. The class comes from a variable, so Tailwind's JIT content scan would purge it.
• The component.channel_visibility and not ... guard means a missing object emits no class, so legacy blocks stay visible everywhere.
• not flag rather than flag == false: Twig's loose == matches null, which would hide every legacy block.
• CSS is used instead of server-side omission so the theme editor's device switcher works. It only resizes the preview iframe, it does not reload it.

Test

1. Set a home component to Desktop only, save.
2. Resize across 768px: the block appears and disappears with no leftover gap.
3. Repeat with a s-block--full-bg block (brands, featured-products-style2) to confirm no empty coloured strip.
4. In the editor, toggle the device switcher: the block reacts without a save or reload.
5. Confirm the existing Hide toggle still hides on both channels.

Stack

Part of the TD-13969 / TD-15122 home-blocks stack. Main PR: Dashboard #27853.

• Components-Module #539
• form-builder-module #233
• ThemeCustomization-Module #626
• Store #11986

This PR carries only the TD-13969 channel-visibility styling; the TD-15122 versions work is backend-only.

[codacy-production]

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[bakrimoharram]

@greptile-apps review