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>

package-lock.json

@@ -10984,6 +10984,36 @@
         "url": "https://opencollective.com/unified"
       }
     },
+    "node_modules/virtua": {
+      "version": "0.48.8",
+      "resolved": "https://registry.npmjs.org/virtua/-/virtua-0.48.8.tgz",
+      "integrity": "sha512-jpsxOw5V4B6hg44JePRLo9DL0TV7N1lBEVtPjKpAJebXyhI2s9lfiXJESaLapNtr3vtiSk/pWHiLf7B2a6UcgQ==",
+      "license": "MIT",
+      "peerDependencies": {
+        "react": ">=16.14.0",
+        "react-dom": ">=16.14.0",
+        "solid-js": ">=1.0",
+        "svelte": ">=5.0",
+        "vue": ">=3.2"
+      },
+      "peerDependenciesMeta": {
+        "react": {
+          "optional": true
+        },
+        "react-dom": {
+          "optional": true
+        },
+        "solid-js": {
+          "optional": true
+        },
+        "svelte": {
+          "optional": true
+        },
+        "vue": {
+          "optional": true
+        }
+      }
+    },
     "node_modules/vite": {
       "version": "5.4.21",
       "dev": true,
@@ -12113,6 +12143,7 @@
         "shiki": "^3.13.0",
         "solid-js": "^1.8.0",
         "solid-toast": "^0.5.0",
+        "virtua": "^0.48.8",
         "yaml": "^2.4.2"
       },
       "devDependencies": {