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/PrimaryButton.module.css

@@ -0,0 +1,17 @@
+.button {
+  width: 100%;
+  padding: 1rem;
+  background: #a8d5e8;
+  border: none;
+  border-radius: 1rem;
+  font-size: 1.25rem;
+  font-weight: 600;
+  cursor: pointer;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.25);
+  transition: transform 80ms ease, box-shadow 80ms ease;
+}
+
+.button:active {
+  transform: translateY(2px);
+  box-shadow: 0 2px 6px rgba(0, 0, 0, 0.2);
+}