feat(12.2-04): add the fileupload field with deferred uploads on the form session key

- sessionKey.ts: one 32-byte base64url key per form mount, sent only in headers
- api/files.ts: FileRoutes over the record and child file routes, XHR upload with progress, 401 refresh and retry
- FileuploadField and FileCaptionModal per UI-SPEC section 3: dropzone, image grid, rows, per-item states, client pre-checks, reorder, protected previews
- FormView provides FORM_SESSION, counts pending changes as dirty and sends X-Session-Key on create and update
- fileupload lang keys in en and pl, admin-spa docs note, deferred smoke test, rebuilt dist

admin/tests/form/registry.test.ts

@@ -39,7 +39,7 @@ describe('field renderer registry (D-05)', () => {
     expect(rendererFor(type)).toBe(component)
   })
 
-  it.each(['colorpicker', 'fileupload', '', 'TEXT', 'constructor'])('falls back to UnsupportedField for %j', (type) => {
+  it.each(['colorpicker', 'codeeditor', '', 'TEXT', 'constructor'])('falls back to UnsupportedField for %j', (type) => {
     expect(rendererFor(type)).toBe(UnsupportedField)
     expect(isRegistered(type)).toBe(false)
   })