Add help modal and back button to score entry pages

HelpModal component shows game setup and scoring rules in a scrollable
overlay, triggered by a ? button top-right on Player Setup and Enter
Scores. Enter Scores also gets a ← back button top-left that returns
to the scoresheet without saving. PageFrame gains a headerActionLeft
slot to support the left-side button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/styles/shared.module.css

@@ -1,3 +1,15 @@
+.helpBtn {
+  background: none;
+  border: none;
+  padding: 0;
+  min-height: unset;
+  font-size: 1.25rem;
+  font-weight: 700;
+  color: #aaa;
+  line-height: 1;
+  box-shadow: none;
+}
+
 .card {
   position: fixed;
   inset: 1rem;