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/NotificationBell.svelte

@@ -67,7 +67,7 @@
 
 <svelte:window onclick={handleWindowClick} onkeydown={(e) => e.key === 'Escape' && (open = false)} />
 
-<div class="relative flex-shrink-0">
+<div class="relative shrink-0">
   <button
     bind:this={buttonRef}
     type="button"
@@ -92,7 +92,7 @@
   {#if open}
     <div
       bind:this={panelRef}
-      class="absolute right-0 z-50 mt-1 flex max-h-[70vh] w-[min(22rem,calc(100vw-2rem))] flex-col rounded border border-border bg-surface shadow"
+      class="absolute right-0 z-50 mt-1 flex max-h-[70vh] w-[min(22rem,calc(100vw-2rem))] flex-col rounded border border-border bg-surface shadow-sm"
       role="dialog"
       aria-label="Notifications"
     >
@@ -123,7 +123,7 @@
                 onclick={() => openItem(item)}
               >
                 <span
-                  class="mt-1.5 h-2 w-2 flex-shrink-0 rounded-full {item.read_at ? '' : 'bg-text-primary'}"
+                  class="mt-1.5 h-2 w-2 shrink-0 rounded-full {item.read_at ? '' : 'bg-text-primary'}"
                   aria-hidden="true"
                 ></span>
                 <span class="min-w-0 flex-1">