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/components/PrimaryButton.module.css

@@ -1,19 +1,13 @@
 .button {
+  composes: pressable from '../styles/shared.module.css';
   width: 100%;
   padding: 1rem;
-  background: #a8d5e8;
+  background: var(--color-blue);
   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:not(:disabled) {
-  transform: translateY(2px);
-  box-shadow: 0 2px 6px rgba(0, 0, 0, 0.2);
 }
 
 .button:disabled {