Show build date next to "Settings" heading

Inject a yyyy-mm-dd_hh:mm timestamp at build time via Vite's `define`
(app/vite.config.ts: __BUILD_DATE__, computed fresh on every
`vite build`/`vite dev` invocation) and render it next to the
Settings panel's title, so it's easy to tell which build is deployed.

- app/vite.config.ts: buildDateString() + define.__BUILD_DATE__
- app/src/vite-env.d.ts: ambient declaration for the injected constant
- app/src/components/SettingsPanel.tsx: render it next to "Settings"
- app/src/styles.css: .build-date styling

Rebuilt and redeployed to the repo root.

app/vite.config.ts

@@ -4,6 +4,13 @@ import { defineConfig, loadEnv, type Plugin } from "vite";
 import react from "@vitejs/plugin-react";
 import { VitePWA } from "vite-plugin-pwa";
 
+/** `yyyy-mm-dd_hh:mm` timestamp of this build/dev-server start, shown in the UI. */
+function buildDateString(): string {
+  const d = new Date();
+  const pad = (n: number) => String(n).padStart(2, "0");
+  return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}_${pad(d.getHours())}:${pad(d.getMinutes())}`;
+}
+
 /** Emits the repo's LICENSE file into the build output. */
 function licensePlugin(): Plugin {
   return {
@@ -27,6 +34,9 @@ export default defineConfig(({ mode }) => {
   const target = env.HERMES_DEV_TARGET;
 
   return {
+    define: {
+      __BUILD_DATE__: JSON.stringify(buildDateString()),
+    },
     server: {
       port: 5173,
       proxy: target