feat(web): add /discover route with search + request flow

Local-input + 250ms debounce drives Lidarr search; tab switch refetches
with new kind. Track-kind Request opens a confirm modal explaining the
album that will be added; Confirm fires createRequest, Cancel is a no-op.
Successful requests flip the card to 'requested' optimistically.
Shell nav now exposes /discover between Search and Playlists.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/src/lib/components/Shell.svelte

@@ -25,6 +25,7 @@
     { href: '/',              label: 'Library' },
     { href: '/library/liked', label: 'Liked' },
     { href: '/search',        label: 'Search' },
+    { href: '/discover',      label: 'Discover' },
     { href: '/playlists',     label: 'Playlists' },
     { href: '/settings',      label: 'Settings' }
   ];