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/api/index.js

@@ -673,6 +673,26 @@ export const employeesApi = {
 // Alias for different casing
 export const businessUnitsApi = businessunitsApi
 
+// Dashboard defaults: visitor-IP -> business-unit mapping for kiosks
+export const dashboardDefaultsApi = {
+  list() {
+    return api.get('/dashboarddefaults')
+  },
+  create(data) {
+    return api.post('/dashboarddefaults', data)
+  },
+  update(id, data) {
+    return api.put(`/dashboarddefaults/${id}`, data)
+  },
+  delete(id) {
+    return api.delete(`/dashboarddefaults/${id}`)
+  },
+  // Resolve the calling display's business unit by its IP
+  visitorLocation() {
+    return api.get('/dashboarddefaults/visitor-location')
+  }
+}
+
 // System Settings API
 export const pluginsApi = {
   list() {