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/fixtures/deferred.form-schema.json

@@ -0,0 +1,68 @@
+{
+  "data": {
+    "name": "Gadget",
+    "fields": [
+      {
+        "name": "name",
+        "label": "Name",
+        "type": "text",
+        "required": true
+      },
+      {
+        "name": "photos",
+        "label": "Photos",
+        "type": "fileupload",
+        "mode": "image",
+        "multiple": true,
+        "maxFiles": 3,
+        "fileTypes": [
+          "jpg",
+          "png"
+        ],
+        "maxFilesize": 2,
+        "thumbOptions": {
+          "mode": "crop"
+        },
+        "useCaption": true
+      }
+    ],
+    "messages": {
+      "create": {
+        "other": "New gadget"
+      },
+      "update": {
+        "other": "Editing gadget"
+      },
+      "saved": {
+        "other": "Gadget „:name” saved"
+      },
+      "deleteConfirm": {
+        "other": "Delete this gadget?"
+      },
+      "deleted": {
+        "other": "Gadget deleted"
+      }
+    },
+    "meta": {
+      "locale": "en"
+    },
+    "assets": {
+      "scripts": [],
+      "styles": []
+    },
+    "redirects": {
+      "create": {
+        "redirect": "acme/demo/gadgets/update/:id",
+        "redirectClose": "acme/demo/gadgets"
+      },
+      "update": {
+        "redirect": "acme/demo/gadgets",
+        "redirectClose": "acme/demo/gadgets"
+      },
+      "default": "acme/demo/gadgets"
+    }
+  },
+  "meta": {
+    "locale": "en"
+  }
+}

admin/tests/fixtures/typed.ts

@@ -10,6 +10,7 @@ import settingsJson from './settings.json'
 import extensionFormSchemaJson from './extension.form-schema.json'
 import extensionListSchemaJson from './extension.list-schema.json'
 import extensionPartialJson from './extension.partial.json'
+import deferredFormSchemaJson from './deferred.form-schema.json'
 import formSchemaJson from './widgets.form-schema.json'
 import listJson from './widgets.list.json'
 import listSchemaJson from './widgets.list-schema.json'
@@ -30,6 +31,8 @@ export const settingsFixture: {
   values: S['cabana.Envelope-cabana_SettingsResult']
 } = settingsJson
 export const formSchemaFixture: S['cabana.Envelope-cabana_FormView'] = formSchemaJson
+/** A gadget form with deferred-binding fields: fileupload (Phase 12.2). */
+export const deferredFormSchemaFixture: S['cabana.Envelope-cabana_FormView'] = deferredFormSchemaJson
 /** A form with a plugin widget and its controller script (Phase 10.1). */
 export const extensionFormSchemaFixture: S['cabana.Envelope-cabana_FormView'] = extensionFormSchemaJson
 /** A list with a header partial and a controller stylesheet (Phase 10.1). */