feat: add user instructions to viewer popup

docs/instructions-audit.md

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+# Instructions UI audit
+
+## Scope
+This audit covers the VidPlex popup instructions banner added in `src/viewer/index.html`, `src/viewer/index.css`, and `src/viewer/index.js`.
+
+## What the UI now explains
+- How to scan the current page for videos
+- How to review clickable results
+- How to open the 2x2 viewer
+- How to clear and rescan
+- A caveat that browser pages, extension pages, and restricted frames may hide videos
+
+## Accessibility checks
+- The instructions section uses `role="region"` and an accessible label
+- The collapse toggle is wired with `aria-expanded` and `aria-controls`
+- The body is hidden with the `hidden` attribute when collapsed
+
+## UX notes
+- The panel is visible on mount so first-time users see the steps immediately
+- The toggle label changes between “Hide instructions” and “Show instructions”
+- The copy intentionally stays short so the popup remains readable in a narrow extension window
+
+## Validation
+- Covered by `__tests__/instructions_ui.test.mjs`
+- Verified alongside the existing build/test flow in the parent implementation task