feat(viewer): add collapsible sidebar

[benvinegar]

Summary

• add an accessible desktop control that collapses the 248px session sidebar into a 40px rail
• preserve the existing mobile drawer at the 700px breakpoint
• keep sidebar collapse state local to each viewer mount and support embedded shadow-root layouts
• add self-hosted and embedded E2E coverage plus a patch changeset

Closes #238.

Screenshots

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Validation

• npm test
• npm run coverage
• npm run test:worker
• npm run typecheck
• npm run lint
• npm run format:check
• npm run security:audit
• focused Chromium E2E tests for desktop collapse, mobile drawer behavior, and embed slots

WebKit could not run locally because its required system libraries are unavailable; CI provides WebKit coverage.

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