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>

webpack.config.js

@@ -113,7 +113,6 @@ module.exports = function (_env = {}, argv = {}) {
     hostedExplorer: "./src/HostedExplorer.tsx",
     terminal: "./src/Terminal/index.ts",
     cellOutputViewer: "./src/CellOutputViewer/CellOutputViewer.tsx",
-    galleryViewer: "./src/GalleryViewer/GalleryViewer.tsx",
     selfServe: "./src/SelfServe/SelfServe.tsx",
     connectToGitHub: "./src/GitHub/GitHubConnector.ts",
     redirectBridge: "./src/redirectBridge.ts",
@@ -154,11 +153,6 @@ module.exports = function (_env = {}, argv = {}) {
       template: "src/CellOutputViewer/cellOutputViewer.html",
       chunks: ["cellOutputViewer"],
     }),
-    new HtmlWebpackPlugin({
-      filename: "gallery.html",
-      template: "src/GalleryViewer/galleryViewer.html",
-      chunks: ["galleryViewer"],
-    }),
     new HtmlWebpackPlugin({
       filename: "connectToGitHub.html",
       template: "src/connectToGitHub.html",