perf: virtualize session message list for long conversations

Replace full DOM rendering with @tanstack/react-virtual to only render
visible messages (~25 DOM nodes instead of N). Wrap SessionMessageItem
in React.memo to prevent unnecessary re-renders on state changes.

package.json

@@ -67,6 +67,7 @@
     "@radix-ui/react-tooltip": "^1.2.8",
     "@radix-ui/react-visually-hidden": "^1.2.4",
     "@tanstack/react-query": "^5.90.3",
+    "@tanstack/react-virtual": "^3.13.23",
     "@tauri-apps/api": "^2.8.0",
     "@tauri-apps/plugin-dialog": "^2.4.0",
     "@tauri-apps/plugin-process": "^2.0.0",