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.

scripts/stage-frontend.mjs

@@ -0,0 +1,61 @@
+// Stage plugin frontends into the Vite tree (ADR-013 Phase 4).
+//
+// Each plugin that owns UI keeps it self-contained under
+// plugins/<name>/frontend/ (routes.js + views/). This script copies the CHOSEN
+// plugins' frontend dirs into frontend/src/.plugins-staged/<name>/ and codegens
+// frontend/src/router/routes.gen.js, which the router imports. A per-site build
+// selects plugins via the SITE_PLUGINS env (comma-separated); with none set,
+// every plugin that has a frontend/ is staged (the default full build).
+//
+// Both outputs are generated (gitignored). Run by npm predev/prebuild.
+
+import { readdirSync, existsSync, rmSync, cpSync, writeFileSync, mkdirSync } from 'fs'
+import { join, dirname } from 'path'
+import { fileURLToPath } from 'url'
+
+const repoRoot = join(dirname(fileURLToPath(import.meta.url)), '..')
+const pluginsDir = join(repoRoot, 'plugins')
+const stagedDir = join(repoRoot, 'frontend', 'src', '.plugins-staged')
+const genFile = join(repoRoot, 'frontend', 'src', 'router', 'routes.gen.js')
+
+const requested = (process.env.SITE_PLUGINS || '')
+  .split(',').map(s => s.trim()).filter(Boolean)
+
+const withFrontend = readdirSync(pluginsDir, { withFileTypes: true })
+  .filter(entry => entry.isDirectory()
+    && existsSync(join(pluginsDir, entry.name, 'frontend', 'routes.js')))
+  .map(entry => entry.name)
+
+const chosen = requested.length
+  ? withFrontend.filter(name => requested.includes(name))
+  : withFrontend
+
+// Copy each chosen plugin's frontend dir into the staging area (clean first so
+// a removed plugin does not linger).
+rmSync(stagedDir, { recursive: true, force: true })
+mkdirSync(stagedDir, { recursive: true })
+for (const name of chosen) {
+  cpSync(join(pluginsDir, name, 'frontend'), join(stagedDir, name),
+    { recursive: true })
+}
+
+// Codegen the aggregation the router imports. `toplevel` is optional per plugin.
+const lines = ['// AUTO-GENERATED by scripts/stage-frontend.mjs - do not edit.']
+const childParts = []
+const topParts = []
+for (const name of chosen) {
+  const id = 'p_' + name.replace(/[^a-zA-Z0-9]/g, '_')
+  lines.push(
+    `import ${id}Default, { toplevel as ${id}Top } from `
+    + `'../.plugins-staged/${name}/routes.js'`)
+  childParts.push(`...(${id}Default || [])`)
+  topParts.push(`...(${id}Top || [])`)
+}
+lines.push('')
+lines.push(`export const stagedChildren = [${childParts.join(', ')}]`)
+lines.push(`export const stagedToplevel = [${topParts.join(', ')}]`)
+lines.push('')
+writeFileSync(genFile, lines.join('\n'))
+
+console.log(`staged ${chosen.length} plugin frontend(s): `
+  + (chosen.join(', ') || '(none)'))