perf: virtualize FormDropdownMenu to reduce DOM nodes and image requests (#8476)

## Summary

Virtualize the FormDropdownMenu to only render visible items, fixing
slow dropdown performance on cloud.

## Changes

- Integrate `VirtualGrid` into `FormDropdownMenu` for virtualized
rendering
- Add computed properties for grid configuration per layout mode
(grid/list/list-small)
- Extend `VirtualGrid` slot to provide original item index for O(1)
lookups
- Change container from `max-h-[640px]` to fixed `h-[640px]` for proper
virtualization

## Review Focus

- VirtualGrid integration within the popover context
- Layout mode switching with `:key="layoutMode"` to force re-render
- Grid style computed properties match original Tailwind classes

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8476-perf-virtualize-FormDropdownMenu-to-reduce-DOM-nodes-and-image-requests-2f86d73d365081b3a79dd5e0b84df944)
by [Unito](https://www.unito.io)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Dropdowns now render with a virtualized grid/list (stable indexes,
responsive sizing) and show an empty-state icon when no items exist.

* **Bug Fixes**
* Reduced layout shift and rendering glitches with improved
spacer/scroll calculations and more reliable media measurement.

* **Style**
* Simplified media rendering (standard img/video), unified item visuals
and hover/background behavior.

* **Tests**
* Added unit and end-to-end tests for virtualization, indexing, layouts,
dynamic updates, and empty states.

* **Breaking Changes**
* Dropdown item/selection shapes and related component props/events were
updated (adapter changes may be required).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: GitHub Action <action@github.com>

src/locales/en/main.json

@@ -2459,7 +2459,8 @@
       "placeholderVideo": "Select video...",
       "placeholderMesh": "Select mesh...",
       "placeholderModel": "Select model...",
-      "placeholderUnknown": "Select media..."
+      "placeholderUnknown": "Select media...",
+      "maxSelectionReached": "Maximum selection limit reached"
     },
     "valueControl": {
       "header": {