fix(web): center nav in window + Library link + Search placeholder

Three operator-reported issues:

- Header was flex with the nav inside a flex-1 span — when the
  right side (search + user menu) grew wider than the left
  (wordmark), the nav's centered position drifted off page-center.
  Switched to grid-cols-3 with justify-self-{start,center,end} so
  the middle column pins to true window-center regardless of side
  widths.
- Library nav link pointed to /library, which 308-redirects via
  +page.server.ts. Operator reported it didn't navigate. Linked the
  nav button directly to /library/artists so SPA navigation skips
  the redirect roundtrip. matchPrefix='/library' keeps isActive
  matching every Library tab.
- SearchInput placeholder was 'Search artists, albums, tracks…' —
  shortened to 'Search' and added a Lucide Search icon inside the
  input on the left. Padding adjusted (pl-7) so the input text
  clears the icon.

web/src/lib/components/SearchInput.svelte

@@ -2,6 +2,7 @@
   import { page } from '$app/state';
   import { goto } from '$app/navigation';
   import { untrack } from 'svelte';
+  import { Search } from 'lucide-svelte';
 
   let value = $state(page.url.searchParams.get('q') ?? '');
   let timeout: ReturnType<typeof setTimeout> | null = null;
@@ -51,12 +52,20 @@
   }
 </script>
 
-<input
-  type="search"
-  placeholder="Search artists, albums, tracks…"
-  aria-label="Search"
-  bind:value
-  oninput={onInput}
-  onkeydown={onKey}
-  class="w-full max-w-md rounded border border-border bg-surface px-3 py-1 text-sm focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
-/>
+<label class="relative block w-full max-w-md">
+  <Search
+    size={14}
+    strokeWidth={1.5}
+    aria-hidden="true"
+    class="pointer-events-none absolute left-2 top-1/2 -translate-y-1/2 text-text-secondary"
+  />
+  <input
+    type="search"
+    placeholder="Search"
+    aria-label="Search"
+    bind:value
+    oninput={onInput}
+    onkeydown={onKey}
+    class="w-full rounded border border-border bg-surface pl-7 pr-3 py-1 text-sm focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+  />
+</label>