Add canvas previews, external extension links, and release notes showcase (#1987)

* Add extension thumbnails and preview assets

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

* Add clickable extension image preview modal

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

* Address PR review feedback

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

* Update website/src/styles/global.css

* Add preview assets for canvas extensions

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

* Update canvas extension preview images

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

* Delete extensions/backlog-swipe-triage/assets/swipe-canvas-triage.png

* Support external canvas extensions and add Coffilot

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

* Add homepage link to GitHub repository

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

* Add release notes showcase canvas extension

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

* Enhance release notes canvas sourcing and layout

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

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Aaron Powell <me@aaron-powell.com>

extensions/release-notes-showcase/README.md

@@ -0,0 +1,7 @@
+# Release Notes Showcase
+
+Interactive canvas for composing, reviewing, and exporting release notes content.
+
+## Assets
+
+- `assets/preview.png` — screenshot preview used by the extensions gallery.