Centralize duplicated card shell, press animation, and color tokens

Extract the rounded-card shell and button press animation into a shared
CSS module composed by PageFrame, TitleScreen, and all buttons, add color
tokens for repeated values, and load the saved game once on mount.

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

src/styles/shared.module.css

@@ -0,0 +1,19 @@
+.card {
+  position: fixed;
+  inset: 1rem;
+  border: 2px solid black;
+  border-radius: 2rem;
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+}
+
+.pressable {
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.25);
+  transition: transform 80ms ease, box-shadow 80ms ease;
+}
+
+.pressable:active:not(:disabled) {
+  transform: translateY(2px);
+  box-shadow: 0 2px 6px rgba(0, 0, 0, 0.2);
+}