tech(pat-upload): lazify pattern, split registration from implementation

pat-upload contributed ~17KB src to the eager patterns chunk on every page,
though the upload UI only appears in folder contents and upload dialogs.

Split it into a thin registration module and upload--implementation.js
holding the jquery/underscore/i18n wiring and the lazily-loaded Dropzone
integration. The thin module keeps registering the .pat-upload trigger and
grafts the implementation prototype on first match. Public options and
emitted markup are unchanged.

pat-structure, pat-relateditems and the contentbrowser app construct upload
imperatively (new Upload(...)) and use the instance right away. The
implementation therefore stays a constructable Base.extend pattern and those
callers import it directly, so new Upload() yields a fully formed instance
synchronously. Registry.register is first-wins, so the eagerly imported thin
module remains the registered .pat-upload pattern; the implementation is
only reached via lazy chunks (the structure app view, the contentbrowser
component and the relateditems dynamic import).

Add a regression test asserting the implementation is constructable and
exposes its methods synchronously.

The implementation now loads only where a .pat-upload element exists.

Author: MrTango

src/pat/contentbrowser/src/ContentBrowser.svelte

@@ -5,7 +5,7 @@
     import { fly } from "svelte/transition";
     import _t from "../../../core/i18n-wrapper";
     import { ensureIntlSupport } from "../../../core/intl-loader";
-    import Upload from "../../upload/upload";
+    import Upload from "../../upload/upload--implementation";
     import contentStore from "./ContentStore";
     import {
         clickOutside,

src/pat/relateditems/relateditems.js

@@ -401,7 +401,7 @@ export default Base.extend({
     },
 
     async initUploadView(disabled) {
-        let Upload = await import("../upload/upload");
+        let Upload = await import("../upload/upload--implementation");
         Upload = Upload.default;
 
         const upload_button = this.$toolbar[0].querySelector(".upload button");

src/pat/structure/js/views/upload.js

@@ -1,7 +1,7 @@
 import $ from "jquery";
 import _ from "underscore";
 import PopoverView from "../../../../core/ui/views/popover";
-import Upload from "../../../upload/upload";
+import Upload from "../../../upload/upload--implementation";
 
 export default PopoverView.extend({
     className: "popover upload",