feat(app): virtualize session timeline

Render SessionDetail through measured dynamic-height virtual rows while preserving disclosure state, UUID navigation, reader anchoring, and tail-follow across live updates.

Add focused state/reconciliation tests plus a production Electron harness covering long-session DOM bounds, scrolling performance, offscreen navigation, and live viewport stability.

app/package-lock.json

@@ -12,6 +12,7 @@
         "chokidar": "^4.0.3"
       },
       "devDependencies": {
+        "@tanstack/vue-virtual": "^3.13.32",
         "@types/better-sqlite3": "^7.6.13",
         "@vitejs/plugin-vue": "^5.0.0",
         "electron": "^33.0.0",
@@ -1839,6 +1840,34 @@
         "node": ">=10"
       }
     },
+    "node_modules/@tanstack/virtual-core": {
+      "version": "3.17.4",
+      "resolved": "https://registry.npmjs.org/@tanstack/virtual-core/-/virtual-core-3.17.4.tgz",
+      "integrity": "sha512-nGm5KteqxasUdThLc2izl6dHUqLv0LQj7Nuyo5gYalTPf/U8a9ermvsl7reT+6ioBW1l8WfpP/mcU338nLXpqw==",
+      "dev": true,
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/tannerlinsley"
+      }
+    },
+    "node_modules/@tanstack/vue-virtual": {
+      "version": "3.13.32",
+      "resolved": "https://registry.npmjs.org/@tanstack/vue-virtual/-/vue-virtual-3.13.32.tgz",
+      "integrity": "sha512-E8OCutx7QnwZdvpJijz0Q2PHsYDWBWjnGr3TvgWiqxTU35jB1kVhtkd93scRV7tTFuId2tg3x2iFiw+IE4evjQ==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@tanstack/virtual-core": "3.17.4"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/tannerlinsley"
+      },
+      "peerDependencies": {
+        "vue": "^2.7.0 || ^3.0.0"
+      }
+    },
     "node_modules/@tootallnate/once": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/@tootallnate/once/-/once-2.0.1.tgz",