Consolidate multi-select modifier detection

## 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 #14765
- [Review comment](https://github.com/Comfy-Org/ComfyUI_frontend/pull/14765#discussion_r3726456497)

Start by locating the existing isMultiSelectKey() helper under renderer/extensions/vueNodes and tracing its Vue node consumers, then inspect MediaAssetCard, MediaVideoTop, and useAssetGridSelection for duplicate modifier checks. Move the helper to a neutral shared location, migrate all named consumers, and add focused tests confirming Ctrl, Cmd, and Shift behavior remains aligned.

Written by the indexing model from the issue text.