refactor: extract reusable library

- move reusable domain, data, state, ui code into src/lib
- update host screens to consume new library exports
- document architecture and configure path aliases
- bump astro integration dependencies for compatibility

Refs #30

src/lib/ui/Layout.module.css

@@ -0,0 +1,51 @@
+.layout {
+  height: 100vh;
+  overflow: hidden;
+  background-color: var(--color-background);
+  color: var(--color-text);
+  display: flex;
+  flex-direction: column;
+}
+
+.content {
+  flex: 1;
+  max-width: 1200px;
+  margin: 0 auto;
+  padding: var(--space-md);
+  width: 100%;
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  min-height: 0;
+}
+
+.screen {
+  width: 100%;
+  height: 100%;
+  display: flex;
+  flex-direction: column;
+  overflow: hidden;
+  min-height: 0;
+}
+
+/* Tablet optimizations */
+@media (min-width: 768px) and (max-width: 1024px) {
+  .content {
+    padding: var(--space-lg) var(--space-xl);
+    max-width: 900px;
+  }
+}
+
+/* Large tablet and small desktop */
+@media (min-width: 1025px) {
+  .content {
+    padding: var(--space-xl);
+  }
+}
+
+/* Mobile adjustments */
+@media (max-width: 767px) {
+  .content {
+    padding: var(--space-sm) var(--space-md);
+  }
+}