Use the git commit hash as the app version

- Spec header, status pill and footer now document '<env> @ <hash>'
  instead of invented release numbers
- Backend settings expose GIT_COMMIT_HASH / APP_ENV / APP_VERSION,
  mirroring the original app
- Frontend bakes the short hash in at build time and shows it in the
  top bar

frontend/vite.config.ts

@@ -1,13 +1,30 @@
+import { execSync } from "node:child_process";
 import { fileURLToPath, URL } from "node:url";
 
 import tailwindcss from "@tailwindcss/vite";
 import react from "@vitejs/plugin-react";
 import { defineConfig } from "vite";
 
+function gitCommitHash(): string {
+  try {
+    return execSync("git rev-parse --short HEAD", {
+      cwd: fileURLToPath(new URL(".", import.meta.url)),
+      stdio: ["ignore", "pipe", "ignore"],
+    })
+      .toString()
+      .trim();
+  } catch {
+    return "unknown";
+  }
+}
+
 const backend = process.env.VITE_BACKEND ?? "http://127.0.0.1:8000";
 
 // https://vite.dev/config/
 export default defineConfig({
+  define: {
+    __GIT_COMMIT_HASH__: JSON.stringify(gitCommitHash()),
+  },
   plugins: [react(), tailwindcss()],
   resolve: {
     alias: {