feat: add version click flash animation with colored easter egg, darken light mode wells

- Version number clicks now trigger copy-flash animation
- After 5 clicks, each additional click flashes a different bright
  saturated color (hot pink, vivid green, electric blue, orange, purple)
- 10th click reveals debug well as before
- Wells in light mode darkened from #f5f5f5 to #e8e8e8 for better
  contrast with white background

Addresses additional requirements from issue #144 comments.

src/popup/styles/main.css

@@ -10,7 +10,7 @@
     --color-border: #000000;
     --color-border-light: #cccccc;
     --color-hover: #eeeeee;
-    --color-well: #f5f5f5;
+    --color-well: #e8e8e8;
     --color-danger-well: #fef2f2;
     --color-section: #dddddd;
 }