Consolidate multi-select modifier detection

[ryan-di]

Summary

Move the shared Ctrl/Cmd/Shift selection-modifier check to a neutral utility and use it across asset and Vue node selection paths.

Why

PR #14765 exposed duplicate event.shiftKey || event.metaKey || event.ctrlKey checks in MediaAssetCard and MediaVideoTop. Those checks need to remain exact complements; if they drift, the same click could both play a video and select the asset.

An existing isMultiSelectKey() helper lives under renderer/extensions/vueNodes, which is not an appropriate dependency for asset components.

Scope

• Choose a neutral shared location for isMultiSelectKey().
• Migrate Vue node selection consumers.
• Migrate MediaAssetCard and MediaVideoTop.
• Migrate the equivalent check in useAssetGridSelection.
• Add focused tests that keep modifier behavior aligned.

References

• PR fix(assets): correct video asset interactions #14765
• Review comment

[coderabbitai]

🔗 Related PRs

#14830 - feat: add persistent generated asset media tabs [open]
#14831 - feat: add local asset file actions [open]
#14948 - fix: drop stale missing-media candidates when the widget value changes [merged]
#14980 - fix: hide local values for linked standard widgets [open]
#14981 - fix: hide linked core media inputs [open]

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