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/FingerprintSettingsCard.svelte

@@ -83,7 +83,7 @@
 
   const inputClass =
     'mt-1 w-28 rounded border border-border bg-background 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';
 </script>
 
 <section class="space-y-4 rounded-xl border border-border bg-surface p-5">
@@ -183,7 +183,7 @@
         class="flex items-start gap-2 rounded-md bg-surface-hover px-3 py-2 text-xs text-text-secondary"
         data-testid="length-warning"
       >
-        <TriangleAlert size={14} class="mt-0.5 flex-shrink-0 text-action-destructive" aria-hidden="true" />
+        <TriangleAlert size={14} class="mt-0.5 shrink-0 text-action-destructive" aria-hidden="true" />
         <span>
           Saving re-fingerprints {libraryTotal > 0
             ? `all ${libraryTotal.toLocaleString()} tracks`
@@ -206,7 +206,7 @@
       <button
         type="button"
         class="rounded-md bg-action-secondary px-4 py-2 text-sm text-action-fg hover:opacity-90
-               focus-visible:outline focus-visible:outline-2 focus-visible:outline-accent
+               focus-visible:outline-solid focus-visible:outline-2 focus-visible:outline-accent
                disabled:cursor-not-allowed disabled:opacity-50"
         disabled={!dirty || saving || problems.length > 0}
         onclick={save}