feat(lazy loading): Implement virtual list with virtua (#241)

### 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>

packages/ui/vite.config.ts

@@ -77,23 +77,23 @@ export default defineConfig({
         theme_color: "#1a1a1a",
       },
       workbox: {
-         // Workbox defaults to 2 MiB; our main bundle can slightly exceed that.
-         // This is a build-time limit for the precache manifest, not a hard runtime cap.
-         maximumFileSizeToCacheInBytes: 3 * 1024 * 1024,
-          // Preserve server-side auth redirects (e.g., /login) instead of serving cached index.html.
-          navigateFallback: null,
-          // Only precache static assets (avoid caching HTML documents / routes).
-          globPatterns: ["**/*.{js,css,png,jpg,jpeg,svg,webp,ico,woff,woff2,ttf,eot,json,webmanifest}"],
-         // Monaco assets can be large; cache them at runtime instead.
-         globIgnores: [
-           "**/*.html",
-           "**/assets/*worker-*.js",
-           "**/assets/editor.api-*.js",
-           "**/monaco/vs/**/*",
-         ],
-         // Only cache static UI assets; never cache API traffic.
-         runtimeCaching: [
-           {
+        // Workbox defaults to 2 MiB; our main bundle can slightly exceed that.
+        // This is a build-time limit for the precache manifest, not a hard runtime cap.
+        maximumFileSizeToCacheInBytes: 3 * 1024 * 1024,
+        // Preserve server-side auth redirects (e.g., /login) instead of serving cached index.html.
+        navigateFallback: null,
+        // Only precache static assets (avoid caching HTML documents / routes).
+        globPatterns: ["**/*.{js,css,png,jpg,jpeg,svg,webp,ico,woff,woff2,ttf,eot,json,webmanifest}"],
+        // Monaco assets can be large; cache them at runtime instead.
+        globIgnores: [
+          "**/*.html",
+          "**/assets/*worker-*.js",
+          "**/assets/editor.api-*.js",
+          "**/monaco/vs/**/*",
+        ],
+        // Only cache static UI assets; never cache API traffic.
+        runtimeCaching: [
+          {
             urlPattern: ({ url, request }) => {
               if (url.pathname.startsWith("/api/")) return false
               if (request.destination === "document") return false