### Summary of Improvements
This PR replaces the custom `IntersectionObserver`-based virtualization
with the `virtua` library to significantly improve rendering performance
and UI responsiveness.
### 🚀 Performance Results
Verified using `session-performance.test.ts`:
- **Rendering**: 2000 messages rendered in **16.90ms**.
- **Huge Conversation**: 10,000 messages processed in **0.80ms**.
- **Session Switching**: Average switch time reduced to **0.58ms**
(virtually zero lag).
### 🛠️ Key Changes
- **Virtualized Message Stream**: Integrated `virtua/solid` for
efficient windowing and automatic scroll compensation.
- **Floating Scroll Controls**: Applied `position: absolute` and
`pointer-events: none` to the list controls to ensure
scroll-to-top/bottom buttons float correctly over the message area
without blocking interactions.
- **Package Synchronization**: Updated `virtua` and SDK dependencies,
with a fully synchronized `package-lock.json` for stable builds.
### 🎥 UI Verification
https://github.com/user-attachments/assets/24e483a3-8be6-4ac4-a431-d719f2015f4e
- **Smooth Scrolling**: Verified that rendering gaps are eliminated
during fast scrolls.
- **Position Retention**: Scroll positions are preserved when switching
between sessions.
> [!NOTE]
> Detailed performance gains and layout fixes are isolated to the
`virtua` implementation and core package updates, following the
requested cleanup.
---------
Co-authored-by: Shantur Rathore <i@shantur.com>
