Test gap: add side-panel render smoke test to prevent blank UI regressions

[arnaudassoumani-collab]

Gap

Current extension E2E tests validate bridge/no-egress behavior but do not include a side-panel render smoke test.

Risk

A runtime crash in sidebar React tree can ship and only be detected manually as a blank/non-usable panel.

Proposal

Add a deterministic E2E smoke test that:

1. Loads the extension.
2. Navigates to chrome-extension://<id>/sidebar.html.
3. Verifies root render + key controls exist.
4. Captures console/page errors and fails on uncaught errors.

Acceptance criteria

• CI fails on uncaught sidebar render exceptions.
• Basic controls are detected in sidebar smoke test.
• Test outputs include captured console/page errors for debugging.