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/components/PageFrame.tsx

@@ -0,0 +1,22 @@
+import styles from './PageFrame.module.css'
+
+interface Props {
+  title: string
+  children: React.ReactNode
+  footer?: React.ReactNode
+}
+
+export default function PageFrame({ title, children, footer }: Props) {
+  return (
+    <div className={styles.frame}>
+      <header className={styles.header}>
+        <h1 className={styles.title}>{title}</h1>
+        <div className={styles.rule} role="separator" />
+      </header>
+      <div className={styles.content}>
+        {children}
+      </div>
+      {footer && <div className={styles.footer}>{footer}</div>}
+    </div>
+  )
+}