feat: add user instructions to viewer popup

docs/instructions-copy.md

@@ -0,0 +1,17 @@
+# Instructions UI copy
+
+## Banner copy
+Quick start
+How to use VidPlex
+
+## Steps
+1. Scan videos — Click Find Videos to search the current browser window for videos.
+2. Review the list — Check the popup list; each found video URL is clickable.
+3. Open the viewer — Click Open Viewer to open the 2x2 grid player in a new window.
+4. Reset and rescan — Click Clear to erase results and scan again.
+
+## Warning copy
+Warning: Browser pages, extension pages, and restricted frames can hide videos, so results may be incomplete.
+
+## Learn-more copy
+Learn more tooltip: Read the VidPlex README and audit notes.