feat: add dark/light mode toggle

- Add blocking inline script in app.html that reads localStorage
  'theme' and applies .dark class before render (no flash)
- Invert gray scale in .dark via CSS custom properties so all
  grayscale colors reverse automatically (light becomes dark)
- Add @custom-variant dark to enable dark: utility classes
- Add border-button toggle ("Dark"/"Light") in TopBar far right
- Persist preference in localStorage

src/app.html

@@ -4,6 +4,16 @@
 <head>
 	<meta charset="utf-8" />
 	<meta name="viewport" content="width=device-width, initial-scale=1" />
+	<script>
+		(function () {
+			const theme = localStorage.getItem('theme');
+			if (theme === 'dark' || (!theme && window.matchMedia('(prefers-color-scheme: dark)').matches)) {
+				document.documentElement.classList.add('dark');
+			} else {
+				document.documentElement.classList.remove('dark');
+			}
+		})();
+	</script>
 	%sveltekit.head%
 </head>