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/QueueTrackRow.svelte

@@ -71,7 +71,7 @@
     hover/focus so the artwork stays legible the rest of the time; the drop
     shadow is what keeps the glyph readable over pale covers without one.
   -->
-  <div class="relative h-10 w-10 flex-shrink-0">
+  <div class="relative h-10 w-10 shrink-0">
     <img
       src={coverUrl(track.album_id)}
       alt=""
@@ -86,7 +86,7 @@
       class="group absolute inset-0 flex cursor-grab items-center justify-center rounded
              text-white/70 transition hover:bg-black/45 hover:text-white
              focus-visible:bg-black/45 focus-visible:text-white
-             focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+             focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-accent"
       style="filter: drop-shadow(0 1px 1px rgb(0 0 0 / 0.9))"
     >
       <GripVertical size={16} />
@@ -101,7 +101,7 @@
   >
     <div class="flex items-center gap-2 min-w-0">
       {#if isCurrent}
-        <span class="text-xs text-accent-fg uppercase tracking-wide flex-shrink-0">Now playing</span>
+        <span class="text-xs text-accent-fg uppercase tracking-wide shrink-0">Now playing</span>
       {/if}
       <span class="font-medium truncate text-text-primary">{track.title}</span>
     </div>
@@ -114,7 +114,7 @@
     type="button"
     onclick={handleRemove}
     aria-label="Remove from queue"
-    class="text-text-secondary hover:text-text-primary flex-shrink-0"
+    class="text-text-secondary hover:text-text-primary shrink-0"
   >
     <X size={16} />
   </button>