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/views/PrintedPartsSettings.vue

@@ -0,0 +1,177 @@
+<template>
+  <div>
+    <div class="page-header">
+      <h2>3D Printed Parts</h2>
+    </div>
+
+    <div class="card form-card">
+      <div v-if="message" class="settings-success">{{ message }}</div>
+      <div v-if="error" class="error-message">{{ error }}</div>
+
+      <div class="form-group">
+        <label>Item code prefix</label>
+        <input v-model="values.printedparts_code_prefix" type="text"
+               class="form-control" maxlength="8" />
+        <p class="field-hint">
+          New items mint codes like {{ values.printedparts_code_prefix || '3DP' }}0042.
+          Changing it does not rename existing items.
+        </p>
+      </div>
+
+      <div class="form-group">
+        <label>Default low-stock threshold</label>
+        <input v-model.number="values.printedparts_default_threshold"
+               type="number" min="0" class="form-control" />
+        <p class="field-hint">Seed value for new items; each item can override.</p>
+      </div>
+
+      <div class="form-group">
+        <label>Unknown badge at the kiosk</label>
+        <select v-model="values.printedparts_unknown_badge" class="form-control">
+          <option value="deny">Deny - refuse badges with no directory match</option>
+          <option value="allow">Allow - record the SSO with no name</option>
+        </select>
+      </div>
+
+      <div class="form-group">
+        <label>Alert shopdb users</label>
+        <div class="user-picker">
+          <label v-for="candidate in users" :key="candidate.userid" class="user-row">
+            <input type="checkbox" :value="String(candidate.userid)"
+                   v-model="selectedUserids" />
+            <span>{{ candidate.username }}</span>
+            <span class="user-email">{{ candidate.email }}</span>
+          </label>
+          <p v-if="users.length === 0" class="field-hint">No users loaded</p>
+        </div>
+        <p class="field-hint">
+          Selected users receive low-stock alerts at their account email.
+        </p>
+      </div>
+
+      <div class="form-group">
+        <label>Alert roles</label>
+        <div class="user-picker">
+          <label v-for="role in roles" :key="role.roleid" class="user-row">
+            <input type="checkbox" :value="String(role.roleid)"
+                   v-model="selectedRoleids" />
+            <span>{{ role.rolename }}</span>
+            <span class="user-email">{{ role.description }}</span>
+          </label>
+          <p v-if="roles.length === 0" class="field-hint">No roles loaded</p>
+        </div>
+        <p class="field-hint">
+          Every active member of a selected role receives low-stock alerts.
+        </p>
+      </div>
+
+      <div class="form-group">
+        <label>Additional alert emails</label>
+        <input v-model="values.printedparts_alert_email" type="text"
+               class="form-control" placeholder="parts-team@example.com, lead@example.com" />
+        <p class="field-hint">
+          Comma-separated. Empty uses the site-wide alert recipients
+          (Settings &gt; System &gt; Email). Alerts fire once when an item
+          crosses its threshold; restocking above re-arms.
+        </p>
+      </div>
+
+      <button class="btn btn-primary" :disabled="saving" @click="save">
+        {{ saving ? 'Saving...' : 'Save' }}
+      </button>
+    </div>
+  </div>
+</template>
+
+<script setup>
+import { ref, onMounted } from 'vue'
+import { settingsApi, usersApi } from '@/api'
+
+const KEYS = [
+  'printedparts_code_prefix',
+  'printedparts_default_threshold',
+  'printedparts_unknown_badge',
+  'printedparts_alert_email',
+  'printedparts_alert_userids',
+  'printedparts_alert_roleids'
+]
+
+const values = ref({
+  printedparts_code_prefix: '3DP',
+  printedparts_default_threshold: 5,
+  printedparts_unknown_badge: 'deny',
+  printedparts_alert_email: '',
+  printedparts_alert_userids: '',
+  printedparts_alert_roleids: ''
+})
+const users = ref([])
+const selectedUserids = ref([])
+const roles = ref([])
+const selectedRoleids = ref([])
+const saving = ref(false)
+const message = ref('')
+const error = ref('')
+
+onMounted(async () => {
+  try {
+    const response = await settingsApi.list({ category: 'printedparts' })
+    const rows = response.data.data || []
+    for (const row of rows) {
+      if (KEYS.includes(row.key)) values.value[row.key] = row.value
+    }
+    values.value.printedparts_default_threshold =
+      parseInt(values.value.printedparts_default_threshold, 10) || 0
+    selectedUserids.value = (values.value.printedparts_alert_userids || '')
+      .split(',').map(id => id.trim()).filter(Boolean)
+    const usersResponse = await usersApi.list()
+    users.value = (usersResponse.data.data || []).filter(
+      candidate => candidate.isactive && candidate.email)
+    selectedRoleids.value = (values.value.printedparts_alert_roleids || '')
+      .split(',').map(id => id.trim()).filter(Boolean)
+    const rolesResponse = await usersApi.roles.list()
+    roles.value = rolesResponse.data.data || []
+  } catch (loadError) {
+    error.value = 'Could not load settings'
+    console.error(loadError)
+  }
+})
+
+async function save() {
+  saving.value = true
+  message.value = ''
+  error.value = ''
+  try {
+    values.value.printedparts_alert_userids = selectedUserids.value.join(',')
+    values.value.printedparts_alert_roleids = selectedRoleids.value.join(',')
+    for (const key of KEYS) {
+      await settingsApi.update(key, String(values.value[key] ?? ''))
+    }
+    message.value = 'Settings saved'
+  } catch (saveError) {
+    error.value = saveError.response?.data?.data?.error?.message || 'Save failed'
+  } finally {
+    saving.value = false
+  }
+}
+</script>
+
+<style scoped>
+.field-hint { color: var(--text-light); font-size: 0.85rem; margin-top: 0.25rem; }
+.user-picker {
+  max-height: 12rem;
+  overflow-y: auto;
+  border: 1px solid var(--border);
+  border-radius: 0.35rem;
+  padding: 0.5rem;
+  display: flex;
+  flex-direction: column;
+  gap: 0.35rem;
+}
+.user-row {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  cursor: pointer;
+}
+.user-email { color: var(--text-light); font-size: 0.85rem; }
+</style>