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/Button.tsx

@@ -0,0 +1,32 @@
+import { h } from 'preact';
+import type { ButtonProps } from '@lib/ui/types';
+import styles from './Button.module.css';
+
+export function Button({ 
+  variant = 'secondary', 
+  size = 'medium', 
+  disabled = false, 
+  children, 
+  onClick,
+  'aria-label': ariaLabel,
+  ...rest 
+}: ButtonProps) {
+  const classNames = [
+    styles.button,
+    styles[variant],
+    styles[size],
+    disabled && styles.disabled,
+  ].filter(Boolean).join(' ');
+
+  return (
+    <button
+      className={classNames}
+      onClick={onClick}
+      disabled={disabled}
+      aria-label={ariaLabel}
+      {...rest}
+    >
+      {children}
+    </button>
+  );
+}