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/routes/artists/[id]/+page.svelte

@@ -114,7 +114,7 @@
         aria-label={`Play ${detail.name}`}
         onclick={onPlay}
         disabled={isStartingPlay || detail.album_count === 0}
-        class="flex h-12 w-12 items-center justify-center rounded-full bg-accent text-text-primary shadow transition-transform hover:scale-105 focus-visible:ring-2 focus-visible:ring-accent disabled:opacity-50 disabled:hover:scale-100"
+        class="flex h-12 w-12 items-center justify-center rounded-full bg-accent text-text-primary shadow-sm transition-transform hover:scale-105 focus-visible:ring-2 focus-visible:ring-accent disabled:opacity-50 disabled:hover:scale-100"
       >
         <Play size={24} strokeWidth={1.5} fill="currentColor" class="ml-0.5" />
       </button>