feat(web): trim main nav; pair Discover+Requests; move chrome to user menu

The main nav was carrying surfaces that didn't belong on it:

- Search led to an empty page when clicked (you only ever want to land
  there from the global SearchInput typing into /search?q=...). Drop it.
- Requests is downstream of Discover (you discover, then you request).
  Lift both onto a shared horizontal tab strip; keep their URLs so
  bookmarks survive. New DiscoverTabs component used by both pages.
- Settings + Admin are operator chrome, not primary surfaces. Move them
  into the username dropdown alongside Log out, with Admin gated on
  is_admin. Users see Settings + Log out; admins see Settings + Admin
  + Log out.

Final main nav: Home / Artists / Albums / Liked / Discover / Playlists.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

web/src/lib/components/DiscoverTabs.svelte

@@ -0,0 +1,36 @@
+<script lang="ts">
+  import { page } from '$app/state';
+
+  type Item = { href: string; label: string };
+
+  // Discover and Requests are paired: discover an artist → request it. The
+  // tab strip lifts both surfaces onto the same horizontal axis so the
+  // operator can move between them without round-tripping through the
+  // main nav.
+  const items: Item[] = [
+    { href: '/discover', label: 'Discover' },
+    { href: '/requests', label: 'Requests' }
+  ];
+
+  function isActive(href: string): boolean {
+    return page.url.pathname.startsWith(href);
+  }
+</script>
+
+<nav aria-label="Discover sections" class="border-b border-border">
+  <ul class="flex gap-2">
+    {#each items as item (item.href)}
+      <li>
+        <a
+          href={item.href}
+          aria-current={isActive(item.href) ? 'page' : undefined}
+          class="block border-b-2 px-3 py-2 text-sm transition-colors {isActive(item.href)
+            ? 'border-accent text-text-primary'
+            : 'border-transparent text-text-secondary hover:text-text-primary'}"
+        >
+          {item.label}
+        </a>
+      </li>
+    {/each}
+  </ul>
+</nav>