fix: replace CSS variable inversion with standard Tailwind dark: classes

The :root.dark CSS variable inversion caused contrast problems -
text-gray-900 and bg-gray-950 both became light in dark mode, making
text invisible on backgrounds.

Fix:
- Remove :root.dark { --color-gray-*: ... } block from layout.css
- Change :root.dark selector to .dark for form element styles
- Add standard Tailwind dark: classes throughout every .svelte page
  - dark:bg-gray-950 (page backgrounds)
  - dark:bg-gray-900 (card/surface backgrounds)
  - dark:bg-gray-800 (table headers, elevated surfaces, active nav links)
  - dark:bg-gray-700 (primary buttons)
  - dark:text-gray-100 (primary text)
  - dark:text-gray-300 (secondary text, labels)
  - dark:text-gray-400 (muted text, inactive nav links, links)
  - dark:border-gray-700 (borders)
  - dark:border-gray-600 (input borders)
  - dark:hover:bg-gray-800 (sidebar hover)
  - dark:hover:bg-gray-600 (button hover)
  - dark:prose-invert (rich text content)
- TopBar: full dark mode support (bg, links, toggle button)
- Sidebar: all nav links, active states, admin submenu get dark mode
- Auth pages (login, register, forgot, reset): full dark mode
- Protected pages (dashboard, account, setup, license-keys,
  proclamations, categories): full dark mode
- Home page, error page, admin page: dark:text-gray-100

src/routes/+error.svelte

@@ -5,9 +5,9 @@
 
 <Window>
 	{#if $page.status === 403}
-		<h1>Not Authorized</h1>
-      <p>
-        You don't have permission to view this page. <a href="/">Go home</a>.
+		<h1 class="dark:text-gray-100">Not Authorized</h1>
+      <p class="dark:text-gray-300">
+        You don't have permission to view this page. <a href="/" class="dark:text-gray-400 dark:hover:text-gray-200">Go home</a>.
       </p>
 	{/if}
 </Window>