chore(deps): Tailwind 4 (#5021)

Renovate's tailwindcss bump (PR #150) plus the migration it needs:

- Theme moves from tailwind.config.js into app.css as `@theme inline`,
  mapping the same FabledSword tokens. tailwind.config.js is gone.
- PostCSS runs @tailwindcss/postcss; autoprefixer is dropped, since
  Tailwind 4 prefixes through Lightning CSS.
- Class renames from @tailwindcss/upgrade 4.3.3, reviewed: outline-none
  -> outline-hidden, focus-visible:outline -> outline-solid, shadow ->
  shadow-sm, shadow-sm -> shadow-xs, flex-shrink-0 -> shrink-0. Bare
  `rounded` stays: v4 keeps it at 0.25rem, as before.
- Three v3 preflight defaults kept in a base layer so nothing changes on
  screen: gray-200 default border colour, gray-400 placeholder text and
  the pointer cursor on buttons.
- The unused class-based dark variant is not carried over; no template
  uses `dark:`.

Clears the five high and two moderate npm audit findings that came in
through Tailwind 3 (braces, chokidar, micromatch, fast-glob,
postcss-selector-parser).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/lib/components/HistoryRow.svelte

@@ -49,7 +49,7 @@
   <img
     src={coverUrl(event.track.album_id)}
     alt=""
-    class="h-11 w-11 flex-shrink-0 rounded object-cover"
+    class="h-11 w-11 shrink-0 rounded object-cover"
     loading="lazy"
   />
   <div class="min-w-0 flex-1">
@@ -58,7 +58,7 @@
       {event.track.artist_name} · {event.track.album_title}
     </div>
   </div>
-  <span title={tooltip} class="flex-shrink-0 text-xs text-text-secondary">
+  <span title={tooltip} class="shrink-0 text-xs text-text-secondary">
     {timestamp}
   </span>
 </button>