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/PlayerBar.svelte

@@ -118,7 +118,7 @@
         <a
           href="/now-playing"
           aria-label="Open now playing"
-          class="flex min-w-0 items-center gap-2 focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+          class="flex min-w-0 items-center gap-2 focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-accent"
           style="flex: 1 1 35%;"
         >
           <img
@@ -153,7 +153,7 @@
               type="button"
               aria-label={player.isPlaying ? 'Pause' : 'Play'}
               onclick={togglePlay}
-              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"
+              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"
             >
               {#if player.state === 'loading' || player.state === 'idle'}
                 <Loader2 data-testid="play-spinner" size={22} strokeWidth={1.5} class="animate-spin" />
@@ -281,7 +281,7 @@
       <a
         href="/now-playing"
         aria-label="Open now playing"
-        class="shrink-0 focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+        class="shrink-0 focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-accent"
       >
         <img
           src={coverUrl(current.album_id)}
@@ -341,7 +341,7 @@
             type="button"
             aria-label={player.isPlaying ? 'Pause' : 'Play'}
             onclick={togglePlay}
-            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"
+            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"
           >
             {#if player.state === 'loading' || player.state === 'idle'}
               <Loader2 size={24} strokeWidth={1.5} class="animate-spin" />