Self-heal a stale cached bundle via a build version

Phones kept running old JS because their cached index.html pointed at an old
hashed asset. Both images now bake the commit SHA as BUILD_VERSION; the backend
exposes it at GET /api/version and the client reloads once with a cache-busting
query when its own baked version differs. Guarded by sessionStorage so it cannot
loop. /api/version is unauthenticated like /api/health.

frontend/vite.config.js

@@ -1,7 +1,22 @@
 import { defineConfig } from 'vite'
 import vue from '@vitejs/plugin-vue'
+import { execSync } from 'node:child_process'
+
+// Short git SHA (or a timestamp fallback) baked into the bundle so the client
+// can detect when the server is running a newer build and reload itself.
+let buildVersion = process.env.VITE_BUILD_VERSION || String(Date.now())
+if (!process.env.VITE_BUILD_VERSION) {
+  try {
+    buildVersion = execSync('git rev-parse --short HEAD').toString().trim()
+  } catch {
+    // not a git checkout; keep the timestamp
+  }
+}
 
 export default defineConfig({
+  define: {
+    __BUILD_VERSION__: JSON.stringify(buildVersion),
+  },
   plugins: [vue()],
   server: {
     port: 5173,