Split Vue into a stable vendor chunk; make sourcemaps opt-out

- Pin `vue` to a named `vendor-vue` chunk (manualChunks) so its hash stays
  stable across app-code changes — returning users keep it cached across
  plugin releases instead of re-downloading ~31 kB gzip each update. Also
  replaces the misleadingly-named `_plugin-vue_export-helper` shared chunk.
- Gate sourcemap emission on AURORA_SOURCEMAP (default on for the local
  hot-patch dev loop; the distribution build sets it to false). Cuts a
  from-scratch dist build's app/dist from ~1.7 MB to ~502 kB by dropping
  27 .map files that ship no value to end users.
- Refresh the stale header comment (was "one entry in this first slice").

Co-Authored-By: Claude Opus 4.8 <[email protected]>

app/vite.config.js

@@ -2,9 +2,20 @@ import { defineConfig } from 'vite';
 import vue from '@vitejs/plugin-vue';
 import { fileURLToPath, URL } from 'node:url';
 
-// Deliberately simple: one entry in this first slice (dashboard). Add more
-// entries here as additional pages get rebuilt — same flat pattern each
-// time, no shared scoping/theme-remap layer to keep in sync across files.
+// Multi-entry build: one entry per admin screen (see rollupOptions.input).
+// Each is enqueued as its own ES module by admin/src/Utility/Assets.php,
+// which also walks the manifest's `imports` chain for shared-chunk CSS.
+// Rollup automatically splits code shared by 2+ entries into its own chunk;
+// `vue` is pinned to a named `vendor-vue` chunk below so its hash stays
+// stable across app-code changes (returning users keep it cached across
+// plugin updates instead of re-downloading it every release).
+//
+// Sourcemaps are on by default for local debugging (the dev loop hot-patches
+// app/dist into a LocalWP install). The distribution build sets
+// AURORA_SOURCEMAP=false so the shipped zip doesn't carry ~1.2 MB of .map
+// files that are useless to end users.
+const emitSourcemaps = process.env.AURORA_SOURCEMAP !== 'false';
+
 export default defineConfig({
   root: fileURLToPath(new URL('./src', import.meta.url)),
   plugins: [vue()],
@@ -16,7 +27,7 @@ export default defineConfig({
   build: {
     outDir: fileURLToPath(new URL('./dist', import.meta.url)),
     emptyOutDir: true,
-    sourcemap: true,
+    sourcemap: emitSourcemaps,
     manifest: true,
     rollupOptions: {
       input: {
@@ -43,6 +54,8 @@ export default defineConfig({
         'bug-report': fileURLToPath(new URL('./src/entries/bug-report.js', import.meta.url)),
       },
       output: {
+        // Keep Vue in its own long-lived vendor chunk (see header note).
+        manualChunks: { 'vendor-vue': ['vue'] },
         entryFileNames: '[name].[hash].js',
         chunkFileNames: 'assets/[name].[hash].js',
         assetFileNames: 'assets/[name].[hash][extname]',