Refactor image carousel: simplify architecture, fix review feedback

- Rename chatImageSlideshow -> imageCarousel as a generic workbench contrib
- Remove editor resolver and collection store (direct EditorInput creation)
- Remove F1 command (single entry point via internal command)
- Fix response matching: only open collection containing clicked image
- Add registerOpenEditorListeners for keyboard/accessibility parity
- Localize all user-facing strings
- Add chat.imageCarousel.enabled preview setting (default: false)
- Rename command to workbench.action.chat.openImageInCarousel
- Use h() helper for declarative DOM, getMediaMime() for MIME types
- Editor not restorable (canSerialize returns false)