feat: Replace vue-virtual-scroller with virtua for chat list virtualization (#13642)

# Pull Request Template

## Description

This PR replaces `vue-virtual-scroller` with
[`virtua`](https://github.com/inokawa/virtua/#benchmark) for the
conversation list virtualization.

### Changes
- Replace `vue-virtual-scroller`
(`DynamicScroller`/`DynamicScrollerItem`) with `virtua`'s `Virtualizer`
component
- Remove `IntersectionObserver`-based infinite scroll in favor of
`Virtualizer`'s `@scroll` event with offset-based bottom detection
- Remove `useEventListener` scroll binding and
`intersectionObserverOptions` computed
- Simplify item rendering — no more `DynamicScrollerItem` wrapper or
`size-dependencies` tracking; `virtua` measures items automatically


## Type of change

- [x] New feature (non-breaking change which adds functionality)


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

package.json

@@ -95,6 +95,7 @@
     "video.js": "7.18.1",
     "videojs-record": "4.5.0",
     "videojs-wavesurfer": "3.8.0",
+    "virtua": "^0.48.6",
     "vue": "^3.5.12",
     "vue-chartjs": "5.3.1",
     "vue-datepicker-next": "^1.0.3",
@@ -103,7 +104,6 @@
     "vue-letter": "^0.2.1",
     "vue-router": "~4.4.5",
     "vue-upload-component": "^3.1.17",
-    "vue-virtual-scroller": "^2.0.0-beta.8",
     "vue3-click-away": "^1.2.4",
     "vuedraggable": "^4.1.0",
     "vuex": "~4.1.0",