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/PlaylistCard.svelte

@@ -184,7 +184,7 @@
   >
     <div
       class="art-wrap relative aspect-square w-full overflow-hidden rounded-md
-             bg-surface-hover shadow-sm transition-all duration-150
+             bg-surface-hover shadow-xs transition-all duration-150
              group-hover:shadow-lg group-hover:ring-1 group-hover:ring-accent/40"
     >
       {#if playlist.cover_url}
@@ -203,7 +203,7 @@
            is often a 2x2 collage; the gradient mutes the collage so
            the playlist NAME becomes the dominant element. -->
       <div
-        class="pointer-events-none absolute inset-0 bg-gradient-to-t
+        class="pointer-events-none absolute inset-0 bg-linear-to-t
                from-black/85 via-black/40 to-transparent"
       ></div>
       <!-- Title burned in at the bottom of the cover. Two lines max;