Add an installable SVG-edit Canvas Extension example

Summary

Add a concrete SVG-edit integration on top of the Canvas Extensions page ABI introduced by OpenHands/OpenHands#16895. The first slice contributes a sidebar-routed SVG Editor page embedding a pinned SVG-edit release and demonstrates local installation, hot enable/disable, and cleanup.

Acceptance criteria

• A Canvas Extension manifest contributes an SVG Editor page.
• The extension mounts a pinned SVG-edit release and removes its DOM on route teardown.
• It can be installed through Customize -> Extensions on a local backend.
• Enabling adds a sidebar route; disabling removes it.
• Automated tests cover the page ABI and cleanup.
• Manual isolated-stack verification is documented in the PR.

Dependencies

• OpenHands/OpenHands#16895
• OpenHands/software-agent-sdk#4395

This issue was created by an AI agent (OpenHands) on behalf of Graham Neubig.

Start with the Canvas Extensions page ABI from OpenHands/OpenHands#16895 and the SDK dependency OpenHands/software-agent-sdk#4395; inspect how manifests define pages and sidebar routes. Verify installation through Customize → Extensions on a local backend, then test enabling/disabling, route teardown, SVG-edit DOM cleanup, and the page ABI. Done means the automated coverage and documented isolated-stack verification satisfy every acceptance criterion.