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/AcoustIDCard.svelte

@@ -125,7 +125,7 @@
   const rowClass = 'flex items-center justify-between gap-4 py-2';
   const inputClass =
     'rounded-md border border-border bg-background px-3 py-1.5 text-sm text-text-primary ' +
-    'placeholder:text-text-muted focus:outline-none focus:ring-2 focus:ring-accent';
+    'placeholder:text-text-muted focus:outline-hidden focus:ring-2 focus:ring-accent';
 </script>
 
 <section class="space-y-4 rounded-xl border border-border bg-surface p-5" data-testid="acoustid-card">
@@ -238,7 +238,7 @@
       <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 || !scoreOk}
         onclick={save}
@@ -293,7 +293,7 @@
           <button
             type="button"
             class="rounded-md border border-border px-4 py-2 text-sm text-text-primary hover:bg-background
-                   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={!overview.ready || overview.status.running || starting}
             title={overview.ready ? undefined : 'Switch the lookup on and add a key first'}