ADR-013 Phase 4: frontend staging machinery + relocate printedparts; fix router crash

The staging step that makes lean per-site frontend builds possible, plus the
first plugin relocated as the pilot.

- scripts/stage-frontend.mjs: copies each chosen plugin's plugins/<name>/frontend/
  into frontend/src/.plugins-staged/<name>/ and codegens routes.gen.js. Plugin
  selection via SITE_PLUGINS (comma-separated); empty = all plugins that have a
  frontend/ (the full build). Wired as npm predev/prebuild; outputs gitignored.
- Router imports routes.gen.js and merges staged routes with the in-tree
  ./routes/*.js glob - dual-location during the transition.
- printedparts relocated: its 6 views (list/detail/form/kiosk + the settings and
  labels views from the shared dirs) moved into plugins/printedparts/frontend/
  views/, core imports rewritten to the @/ alias; routes.js is the self-contained
  route module. Its old in-tree route file is removed.

Also fixes a crash the previous commit (37c764b) shipped: slides.js exports only
`toplevel` (its child routes live in core.js), so the router's
flatMap(m => m.default) produced an undefined child and threw
"Cannot read properties of undefined (reading 'path')" at load - the whole SPA
went blank. Guarded with `m.default || []`. (The earlier "print pages are blank"
reading was this crash, not page nature.)

Verified live: /machines renders again; the relocated /printedparts list renders
identically from the staged plugin frontend; SITE_PLUGINS=machines excludes
printedparts from routes.gen. Build (via npm, runs stage) + vitest + naming green.

plugins/printedparts/frontend/routes.js

@@ -0,0 +1,58 @@
+/**
+ * Printedparts plugin frontend routes (ADR-013 Phase 4: self-contained plugin
+ * frontend). Views live beside this file under views/; core imports use the
+ * @/ alias. The stage-frontend step copies this whole frontend/ dir into the
+ * Vite tree and aggregates these routes, so a per-site build that omits
+ * printedparts carries none of this code.
+ *
+ * `default` = AppLayout child routes; `toplevel` = full-screen routes.
+ */
+export default [
+  {
+    path: 'printedparts',
+    name: 'printedparts',
+    component: () => import('./views/PrintedItemsList.vue'),
+    meta: { requiresAuth: true, plugin: 'printedparts' }
+  },
+  {
+    path: 'printedparts/new',
+    name: 'printedparts-new',
+    component: () => import('./views/PrintedItemForm.vue'),
+    meta: { requiresAuth: true, plugin: 'printedparts' }
+  },
+  {
+    path: 'printedparts/:id',
+    name: 'printedparts-detail',
+    component: () => import('./views/PrintedItemDetail.vue'),
+    meta: { requiresAuth: true, plugin: 'printedparts' }
+  },
+  {
+    path: 'printedparts/:id/edit',
+    name: 'printedparts-edit',
+    component: () => import('./views/PrintedItemForm.vue'),
+    meta: { requiresAuth: true, plugin: 'printedparts' }
+  },
+  {
+    path: 'settings/printedparts',
+    name: 'settings-printedparts',
+    component: () => import('./views/PrintedPartsSettings.vue'),
+    meta: { requiresAuth: true, requiresAdmin: true, plugin: 'printedparts' }
+  }
+]
+
+export const toplevel = [
+  {
+    // Touch kiosk for taking 3D-printed parts: scan bin, scan badge, keypad.
+    path: '/parts-kiosk',
+    name: 'parts-kiosk',
+    component: () => import('./views/PartsKiosk.vue'),
+    meta: { plugin: 'printedparts' }
+  },
+  {
+    // Requires login: lists the whole catalog, printedparts.view-gated at the API.
+    path: '/print/printedparts-labels',
+    name: 'print-printedparts-labels',
+    component: () => import('./views/PrintedPartsLabels.vue'),
+    meta: { requiresAuth: true, plugin: 'printedparts' }
+  }
+]