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/DiscoverResultCard.svelte

@@ -86,7 +86,7 @@
       <button
         type="button"
         aria-label={`Bring ${title} back`}
-        class="rounded-md border border-border px-3 py-1.5 text-sm text-text-primary hover:bg-surface-hover focus:outline-none focus:ring-2 focus:ring-accent"
+        class="rounded-md border border-border px-3 py-1.5 text-sm text-text-primary hover:bg-surface-hover focus:outline-hidden focus:ring-2 focus:ring-accent"
         onclick={() => onUnsnooze?.()}
       >
         Undo
@@ -112,7 +112,7 @@
             type="button"
             aria-label={`Not right now — hide ${title} for a while`}
             title="Not right now"
-            class="rounded-md border border-border p-1.5 text-text-secondary hover:bg-surface-hover hover:text-text-primary focus:outline-none focus:ring-2 focus:ring-accent"
+            class="rounded-md border border-border p-1.5 text-text-secondary hover:bg-surface-hover hover:text-text-primary focus:outline-hidden focus:ring-2 focus:ring-accent"
             onclick={() => onSnooze?.()}
           >
             <Clock size={16} strokeWidth={1} />