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/Shell.svelte

@@ -94,13 +94,13 @@
     </nav>
 
     <div class="flex items-center gap-1 md:gap-3 justify-self-end min-w-0">
-    <div class="hidden md:block min-w-0 max-w-xs flex-shrink">
+    <div class="hidden md:block min-w-0 max-w-xs shrink">
       <SearchInput />
     </div>
 
     <NotificationBell />
 
-    <div class="relative flex-shrink-0">
+    <div class="relative shrink-0">
       <button
         bind:this={menuButtonRef}
         type="button"
@@ -120,7 +120,7 @@
       {#if menuOpen}
         <div
           bind:this={menuRef}
-          class="absolute right-0 z-50 mt-1 w-40 rounded border border-border bg-surface shadow"
+          class="absolute right-0 z-50 mt-1 w-40 rounded border border-border bg-surface shadow-sm"
           role="menu"
           tabindex="-1"
         >