fix(client): improve render code accessibility

[vansh-nagar]

Summary

Improves keyboard and screen-reader behavior for the Render Code overlay in the Components inspector.

• exposes the overlay as a labelled dialog
• moves focus to the close action when the overlay opens
• closes the overlay with Escape
• restores focus to the invoking control when the overlay unmounts
• adds an integration test covering semantics, focus, Escape, and focus restoration

Relates to #885.

Validation

• pnpm vitest run tests/integration/client/render-code.test.ts
• pnpm typecheck
• pnpm lint
• pnpm format:check
• pnpm --filter @vue/devtools-client build

The local pre-commit dependency check could not run fully offline because one tarball was absent from the pnpm store; the same lint and formatting commands enforced by the hook were run directly and passed.

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