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.test.ts

@@ -0,0 +1,35 @@
+import { describe, expect, test, vi } from 'vitest';
+import { render, screen } from '@testing-library/svelte';
+
+const state = vi.hoisted(() => ({
+  pageUrl: new URL('http://localhost/discover')
+}));
+
+vi.mock('$app/state', () => ({
+  page: { get url() { return state.pageUrl; } }
+}));
+
+import DiscoverTabs from './DiscoverTabs.svelte';
+
+describe('DiscoverTabs', () => {
+  test('Discover tab is current on /discover', () => {
+    state.pageUrl = new URL('http://localhost/discover');
+    render(DiscoverTabs);
+    expect(screen.getByRole('link', { name: /discover/i })).toHaveAttribute('aria-current', 'page');
+    expect(screen.getByRole('link', { name: /requests/i })).not.toHaveAttribute('aria-current');
+  });
+
+  test('Requests tab is current on /requests', () => {
+    state.pageUrl = new URL('http://localhost/requests');
+    render(DiscoverTabs);
+    expect(screen.getByRole('link', { name: /requests/i })).toHaveAttribute('aria-current', 'page');
+    expect(screen.getByRole('link', { name: /discover/i })).not.toHaveAttribute('aria-current');
+  });
+
+  test('renders both tabs in order', () => {
+    state.pageUrl = new URL('http://localhost/discover');
+    render(DiscoverTabs);
+    const labels = screen.getAllByRole('link').map((l) => l.textContent?.trim());
+    expect(labels).toEqual(['Discover', 'Requests']);
+  });
+});