Add PageFrame, PrimaryButton, and shared layout for non-title pages

- PageFrame: parchment-background frame with title, diamond-rule divider,
  scrollable content area, and optional bottom-pinned footer slot
- PrimaryButton: shared full-width button (light blue, drop shadow, pressed
  state via translateY + shadow reduction) — used by TitleScreen and PlayerSetup
- Global button base in index.css ensures touch-friendly sizing everywhere
- TitleScreen consolidated to use PrimaryButton instead of its own button style

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

src/pages/ScoreSheet.tsx

@@ -1,3 +1,5 @@
+import PageFrame from '../components/PageFrame'
+
 interface Props {
   onScoreRound: () => void
   onNewGame: () => void
@@ -5,10 +7,9 @@ interface Props {
 
 export default function ScoreSheet({ onScoreRound, onNewGame }: Props) {
   return (
-    <div>
-      <h1>Score Sheet</h1>
+    <PageFrame title="Score Sheet">
       <button onClick={onScoreRound}>Score Round</button>
       <button onClick={onNewGame} style={{ fontSize: '0.75em' }}>New Game</button>
-    </div>
+    </PageFrame>
   )
 }