Add dashboard defaults (visitor-IP -> business-unit) for kiosk displays

Classic feature gap: a shopfloor/lobby kiosk auto-selects which business unit to
show based on the display PC's IP (classic dashboarddefaults table +
apivisitorlocation.asp). For the main admin dashboard this does nothing - it is
kiosk/visitor-display infra.

- Model: DashboardDefault (dashboarddefaults: ipaddress unique, businessunitid
  FK, description). Migration 7d01_dashboarddefaults (head).
- API (core, /api/dashboarddefaults): CRUD + GET /visitor-location that resolves
  the calling display's business unit from its IP (X-Forwarded-For/remote_addr,
  or explicit ?ipaddress=); unmapped IP returns a null businessunitid, not an
  error. Unauthenticated resolve (kiosks); writes are admin.
- Frontend: ShopfloorDashboard auto-selects its business unit via visitor-location
  on load when none is chosen; Settings > Dashboard Defaults CRUD page +
  dashboardDefaultsApi client.

Tests: create + resolve by IP -> BU; unmapped IP -> null; duplicate IP 409.
191 tests pass, naming green, app boots, endpoint + admin page verified live.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/views/settings/SettingsIndex.vue

@@ -57,6 +57,12 @@
         <p>Manage organizational units</p>
       </router-link>
 
+      <router-link to="/settings/dashboarddefaults" class="settings-card">
+        <div class="card-icon"><MonitorSmartphone :size="28" /></div>
+        <h3>Dashboard Defaults</h3>
+        <p>Map kiosk IPs to a default business unit</p>
+      </router-link>
+
       <router-link to="/settings/vlans" class="settings-card">
         <div class="card-icon"><Globe :size="28" /></div>
         <h3>VLANs</h3>
@@ -97,7 +103,7 @@
 </template>
 
 <script setup>
-import { Factory, MapPin, Tag, Package, Droplets, Monitor, Laptop, Cog, Building, Globe, Link, Settings, FileText, Users, Puzzle } from 'lucide-vue-next'
+import { Factory, MapPin, Tag, Package, Droplets, Monitor, MonitorSmartphone, Laptop, Cog, Building, Globe, Link, Settings, FileText, Users, Puzzle } from 'lucide-vue-next'
 </script>
 
 <style scoped>