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.

frontend/package.json

@@ -4,7 +4,10 @@
   "private": true,
   "type": "module",
   "scripts": {
+    "stage": "node ../scripts/stage-frontend.mjs",
+    "predev": "npm run stage",
     "dev": "vite",
+    "prebuild": "npm run stage",
     "build": "vite build",
     "preview": "vite preview",
     "test": "vitest run",