feat(web): add Library/Search/Playlists placeholder pages

Library replaces the scaffold splash. Search and Playlists are routable
so the Shell's sidebar nav works end-to-end before the real features land.

web/src/routes/+page.svelte

@@ -1,8 +1,2 @@
-<main class="flex h-screen items-center justify-center">
-  <div class="text-center">
-    <h1 class="text-4xl font-semibold">Minstrel</h1>
-    <p class="mt-2 text-text-secondary">
-      Scaffold &mdash; UI features land in subsequent plans.
-    </p>
-  </div>
-</main>
+<h1 class="text-2xl font-semibold">Library</h1>
+<p class="mt-2 text-text-secondary">Library views land in a subsequent plan.</p>

web/src/routes/playlists/+page.svelte

@@ -0,0 +1,2 @@
+<h1 class="text-2xl font-semibold">Playlists</h1>
+<p class="mt-2 text-text-secondary">Playlists land in a subsequent plan.</p>

web/src/routes/search/+page.svelte

@@ -0,0 +1,2 @@
+<h1 class="text-2xl font-semibold">Search</h1>
+<p class="mt-2 text-text-secondary">Search lands in a subsequent plan.</p>