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/PlaylistTrackRow.svelte

@@ -103,8 +103,8 @@
       aria-label="Reorder track (drag or use arrow keys)"
       aria-keyshortcuts="ArrowUp ArrowDown"
       onkeydown={handleHandleKeydown}
-      class="flex-shrink-0 cursor-grab text-text-muted hover:text-text-primary
-             active:cursor-grabbing focus-visible:outline focus-visible:outline-2
+      class="shrink-0 cursor-grab text-text-muted hover:text-text-primary
+             active:cursor-grabbing focus-visible:outline-solid focus-visible:outline-2
              focus-visible:outline-accent disabled:cursor-not-allowed disabled:opacity-50"
     >
       <GripVertical size={14} strokeWidth={1} />
@@ -129,7 +129,7 @@
     </div>
   </button>
 
-  <span class="flex-shrink-0 text-xs text-text-muted">{format(row.duration_sec)}</span>
+  <span class="shrink-0 text-xs text-text-muted">{format(row.duration_sec)}</span>
 
   {#if !isUnavailable && liveTrack}
     <LikeButton entityType="track" entityId={liveTrack.id} />
@@ -141,7 +141,7 @@
       type="button"
       onclick={() => onRemove(row.position)}
       aria-label={`Remove ${row.title} from playlist`}
-      class="flex-shrink-0 rounded p-1 text-text-muted hover:bg-surface hover:text-action-destructive"
+      class="shrink-0 rounded p-1 text-text-muted hover:bg-surface hover:text-action-destructive"
     >
       <X size={14} strokeWidth={1} />
     </button>