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/library/genres/+page.svelte

@@ -195,7 +195,7 @@
               bind:value={sortMode}
               aria-label="Sort genres"
               class="rounded border border-border bg-surface px-2 py-1 text-sm text-text-primary
-                     focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent"
+                     focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-accent"
             >
               <option value="count">Most tracks</option>
               <option value="name">A–Z</option>
@@ -242,7 +242,7 @@
                      focus-visible:ring-2 focus-visible:ring-accent"
             >
               <span class="truncate text-text-primary">{g.genre}</span>
-              <span class="flex-shrink-0 text-sm text-text-secondary">
+              <span class="shrink-0 text-sm text-text-secondary">
                 {g.track_count}
               </span>
             </a>