Remove gallery.html and all associated gallery functionality

Remove the standalone gallery.html entry point, the in-app gallery tab,
the publish-to-gallery pane, and all gallery-related components, utilities,
and API methods that are no longer needed.

Deleted:
- src/GalleryViewer/ (standalone entry point)
- src/Explorer/Controls/NotebookGallery/ (gallery components)
- src/Explorer/Controls/Header/GalleryHeaderComponent.tsx
- src/Explorer/Tabs/GalleryTab.tsx
- src/Explorer/Panes/PublishNotebookPane/ (publish pane)
- src/Utils/GalleryUtils.ts and tests
- images/GalleryIcon.svg

Edited:
- webpack.config.js (removed entry point and HTML plugin)
- Explorer.tsx (removed openGallery, publishNotebook methods)
- ResourceTreeAdapter.tsx (removed gallery tree node and publish menu)
- NotebookV2Tab.ts (removed publish-to-gallery button)
- NotebookManager.tsx (removed openPublishNotebookPane)
- NotebookViewerComponent.tsx (stripped gallery actions)
- JunoClient.ts (removed gallery interfaces and API methods)
- TelemetryConstants.ts, Constants.ts, ViewModels.ts,
  extractFeatures.ts, useKnockoutExplorer.ts (removed gallery constants)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/hooks/useKnockoutExplorer.ts

@@ -1008,9 +1008,6 @@ function updateContextsFromPortalMessage(inputs: DataExplorerInputsFrame) {
     if (inputs.flights.indexOf(Flights.NotebooksDownBanner) !== -1) {
       userContext.features.notebooksDownBanner = true;
     }
-    if (inputs.flights.indexOf(Flights.PublicGallery) !== -1) {
-      userContext.features.publicGallery = true;
-    }
   }
 
   // Handle initial theme from portal