feat(frontend): scaffold SvelteKit + Tailwind with landing (part 1)

New SvelteKit app in frontend/ (minimal template, TypeScript, TailwindCSS v4).
Adds the app shell (dark music theme navbar + footer) and a landing page with
hero and feature cards. CTAs point to /login and /explore (built in later parts).

Verified: production build succeeds and the page renders with Tailwind styles
applied (checked via preview).

frontend/src/app.html

@@ -0,0 +1,12 @@
+<!doctype html>
+<html lang="en">
+	<head>
+		<meta charset="utf-8" />
+		<meta name="viewport" content="width=device-width, initial-scale=1" />
+		<meta name="text-scale" content="scale" />
+		%sveltekit.head%
+	</head>
+	<body data-sveltekit-preload-data="hover">
+		<div style="display: contents">%sveltekit.body%</div>
+	</body>
+</html>