fix: improve dark mode contrast for wells and balance display

- Change dark mode --color-well from #0a0a0a to #111111 for visible
  contrast against #000000 background
- Add explicit text-fg class to balance display element to ensure
  white text in dark mode

src/popup/styles/main.css

@@ -22,7 +22,7 @@ html.dark {
     --color-border: #ffffff;
     --color-border-light: #333333;
     --color-hover: #111111;
-    --color-well: #0a0a0a;
+    --color-well: #111111;
     --color-danger-well: #1a0000;
     --color-section: #222222;
 }