fix(go): align adults cell class names with Python; un-underline Pay buttons

- Map unpaid|partial → cell-unpaid (or cell-unpaid-current for current
  month) and surplus → cell-overridden, matching Python's Jinja logic;
  avoids emitting non-existent cell-partial/cell-surplus classes that
  caused Pay buttons to escape the table.
- Add text-decoration: none to .pay-btn so anchor-based Pay links don't
  show the default underline.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

go/internal/web/static/css/app.css

@@ -177,6 +177,7 @@ tr:hover {
     transform: translateY(-50%);
     background: #ff3333;
     color: white;
+    text-decoration: none;
     border: none;
     border-radius: 3px;
     padding: 2px 6px;