feat: integrate photosphere viewer with Nextcloud Viewer app (OCA.Viewer)

[Copilot]

The app previously rendered photosphere images in a custom full-page iframe overlay. This integrates it into the Nextcloud universal Viewer app (OCA.Viewer) so photospheres open inside the standard viewer modal.

New: Vue component + viewer handler registration

src/components/PhotoSphereViewer.vue — Vue 3 component wrapping @photo-sphere-viewer/core directly (no iframe). Reads XMP metadata (cropping config etc.) from the files-photospheres-xmp-metadata DAV attribute already present on the file Node, and renders the panorama into an inline <div>.

src/viewerHandler.js — Registers the component with OCA.Viewer on load:

window.OCA.Viewer.registerHandler({
  id: 'photosphere',
  mimes: ['image/jpeg'],
  group: null,           // no prev/next between photospheres yet
  component: PhotoSphereViewerComponent,
  canHandleFile(file) {
    const meta = getDavXmpMeta(file)
    return !!(meta && (meta.usePanoramaViewer === true || meta.usePanoramaViewer === 1))
  },
})

canHandleFile ensures regular JPEGs fall through to the default image viewer — only files carrying photosphere XMP metadata are intercepted.

Modified: fileAction.js delegate to OCA.Viewer

_actionHandler now calls OCA.Viewer.open({ path, fileInfo }) when OCA.Viewer is present, which triggers the registered component above. The iframe path is retained as a fallback for environments where the viewer app is absent.

Build / PHP wiring

• webpack.js: added vue-loader rule + VueLoaderPlugin; viewerHandler added as an entry point.
• AddScriptsAndStylesListener.php: loads viewerHandler after the viewer app (same dependency ordering as fileAction).
• package.json: vue-loader ^17 as devDependency.
• Listener unit test updated to assert viewerHandler is also enqueued.