fix: style private key as red well, remove explicit copy text

- Replace dashed border with light red well (bg-danger-well) and rounded corners
- Remove redundant 'Click to copy.' paragraph
- Add --color-danger-well theme token

src/popup/styles/main.css

@@ -11,6 +11,7 @@
     --color-border-light: #cccccc;
     --color-hover: #eeeeee;
     --color-well: #f5f5f5;
+    --color-danger-well: #fef2f2;
     --color-section: #dddddd;
 }