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.

pnpm-lock.yaml

@@ -89,6 +89,9 @@ importers:
       '@tanstack/react-query':
         specifier: ^5.90.3
         version: 5.90.3(react@18.3.1)
+      '@tanstack/react-virtual':
+        specifier: ^3.13.23
+        version: 3.13.23(react-dom@18.3.1(react@18.3.1))(react@18.3.1)
       '@tauri-apps/api':
         specifier: ^2.8.0
         version: 2.8.0
@@ -1468,6 +1471,15 @@ packages:
     peerDependencies:
       react: ^18 || ^19
 
+  '@tanstack/react-virtual@3.13.23':
+    resolution: {integrity: sha512-XnMRnHQ23piOVj2bzJqHrRrLg4r+F86fuBcwteKfbIjJrtGxb4z7tIvPVAe4B+4UVwo9G4Giuz5fmapcrnZ0OQ==}
+    peerDependencies:
+      react: ^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0
+      react-dom: ^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0
+
+  '@tanstack/virtual-core@3.13.23':
+    resolution: {integrity: sha512-zSz2Z2HNyLjCplANTDyl3BcdQJc2k1+yyFoKhNRmCr7V7dY8o8q5m8uFTI1/Pg1kL+Hgrz6u3Xo6eFUB7l66cg==}
+
   '@tauri-apps/api@2.8.0':
     resolution: {integrity: sha512-ga7zdhbS2GXOMTIZRT0mYjKJtR9fivsXzsyq5U3vjDL0s6DTMwYRm0UHNjzTY5dh4+LSC68Sm/7WEiimbQNYlw==}
 
@@ -4275,6 +4287,14 @@ snapshots:
       '@tanstack/query-core': 5.90.3
       react: 18.3.1
 
+  '@tanstack/react-virtual@3.13.23(react-dom@18.3.1(react@18.3.1))(react@18.3.1)':
+    dependencies:
+      '@tanstack/virtual-core': 3.13.23
+      react: 18.3.1
+      react-dom: 18.3.1(react@18.3.1)
+
+  '@tanstack/virtual-core@3.13.23': {}
+
   '@tauri-apps/api@2.8.0': {}
 
   '@tauri-apps/cli-darwin-arm64@2.8.1':