fix(dashboard): refresh Sessions list in real time when new sessions are created

The dashboard's FastAPI server and a terminal CLI are separate processes
sharing one SQLite session DB; there is no inter-process push channel.
The Sessions page polled the 50 newest sessions every 5s for the
"overview" card but only re-fetched the paginated sessions list on page
change or delete, so a session started in a terminal never appeared in
the list until the user navigated.

Reuse the existing 5s overview poll as a change signal: when the head
session id changes, silently reload the current page (no loading
spinner flicker, no scroll/reset of expanded rows or bulk selection,
which are keyed by id). The detection logic is extracted into a pure
shouldRefreshSessions() helper with unit tests. Adds a minimal vitest
setup for web/ (test script + config).

web/vitest.config.ts

@@ -0,0 +1,16 @@
+import { defineConfig } from "vitest/config";
+import react from "@vitejs/plugin-react";
+import path from "path";
+
+export default defineConfig({
+  plugins: [react()],
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+  test: {
+    environment: "node",
+    include: ["src/**/*.test.{ts,tsx}"],
+  },
+});