Associate Decoy Mode help text and announce popup status updates accessibly

[alex-w-developer]

Problem

At main 332c66f, the popup switch has an accessible name, but no aria-describedby connection to its existing explanatory paragraph. The footer status is also a plain paragraph: asynchronous error and mode-warning text changes have no live-region semantics.

These are concrete markup gaps; screen-reader behavior has not yet been manually tested.

Starting points

• popup/popup.html: #decoy-mode-toggle, #decoy-mode-description, and #status-line
• popup/popup.js: renderReport, loadReport, and handleDecoyModeChange

Acceptance criteria

• Associate the switch with the existing explanatory text using accessible description markup.
• Make relevant status changes announce politely without moving keyboard focus.
• Preserve the switch's accessible name, checked state, keyboard operation, and current visual layout.
• Avoid placing all statistics or the entire popup in a live region.
• Verify with keyboard and a screen reader: toggle description, successful mode change warning, and a simulated update failure.
• Run npm run check; document the manual accessibility verification in the PR.

Scope and overlap

This is a small accessibility change using existing text. #19 owns global-versus-per-site Decoy Mode behavior and wording; do not redesign its scope here. No extension permissions or rule changes are needed.

New contributor notes

This is intended as a small first contribution. Start with the files above and open a focused PR referencing this issue. No assignee has been reserved.