#5 ● Add CSV import feature to the Customer create screen

- Upload a CSV, auto-match columns to customer type attributes,
    manually assign any unmatched fields via dropdown, preview all
    rows (ok / duplicate / empty) before any data is written, then
    approve to bulk-insert with per-row error reporting

  Files changed:
  - routes/web.php: three new POST routes (upload, preview, approve)
  - CustomerController: importUpload / importPreview / importApprove
    methods + applyMapping / fileImport helpers; reuses FileImportService
  - customers/create.php: x-data moved to content-stack; CSV import
    panel (idle → mapping → preview → done) shown after type selection
  - app.js: customerForm extended with 14 state properties and 10 methods
    (auto-match, unused-header dropdowns, upload/preview/approve flows)
  - site.css: mapping table, scrollable preview table, status badges,
    preview stats bar using existing design tokens