Add dark mode with theme toggle and OpenSpec change

- Add @custom-variant dark in globals.css for class-based dark mode
- Add ThemeToggle component with localStorage persistence and system preference fallback
- Inject blocking inline script in layout to prevent flash on load
- Apply dark: variants across all components (layout, site-nav, home-dashboard, expense-workspace, paycheck-workspace, recurring-expense-manager) and page headers
- Create openspec/changes/theming-dark-mode with proposal, design, and tasks artifacts

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/add-expense/page.tsx

@@ -10,9 +10,9 @@ export default function AddExpensePage() {
   return (
     <div className="space-y-8">
       <header className="max-w-2xl space-y-3">
-        <p className="text-sm font-semibold uppercase tracking-[0.28em] text-amber-700">Add Expense</p>
-        <h1 className="text-4xl font-semibold text-stone-950">Capture spending while it still feels fresh.</h1>
-        <p className="text-lg leading-8 text-stone-600">
+        <p className="text-sm font-semibold uppercase tracking-[0.28em] text-amber-700 dark:text-amber-500">Add Expense</p>
+        <h1 className="text-4xl font-semibold text-stone-950 dark:text-white">Capture spending while it still feels fresh.</h1>
+        <p className="text-lg leading-8 text-stone-600 dark:text-stone-400">
           Enter the shop name and the app can auto-fill a category locally for known merchants, with offline AI help for unfamiliar ones.
         </p>
       </header>