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/components/theme-toggle.tsx

@@ -0,0 +1,44 @@
+"use client";
+
+import { useEffect, useState } from "react";
+
+export function ThemeToggle() {
+  const [isDark, setIsDark] = useState(false);
+
+  useEffect(() => {
+    setIsDark(document.documentElement.classList.contains("dark"));
+  }, []);
+
+  function toggle() {
+    const root = document.documentElement;
+    if (root.classList.contains("dark")) {
+      root.classList.remove("dark");
+      localStorage.setItem("theme", "light");
+      setIsDark(false);
+    } else {
+      root.classList.add("dark");
+      localStorage.setItem("theme", "dark");
+      setIsDark(true);
+    }
+  }
+
+  return (
+    <button
+      type="button"
+      onClick={toggle}
+      aria-label={isDark ? "Switch to light mode" : "Switch to dark mode"}
+      className="rounded-full border border-stone-300/80 bg-white/80 px-3 py-2 text-stone-700 transition hover:border-stone-900 hover:text-stone-950 dark:border-stone-600 dark:bg-stone-800 dark:text-stone-300 dark:hover:border-stone-400 dark:hover:text-white"
+    >
+      {isDark ? (
+        <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <circle cx="12" cy="12" r="4" />
+          <path d="M12 2v2M12 20v2M4.93 4.93l1.41 1.41M17.66 17.66l1.41 1.41M2 12h2M20 12h2M6.34 17.66l-1.41 1.41M19.07 4.93l-1.41 1.41" />
+        </svg>
+      ) : (
+        <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="2" strokeLinecap="round" strokeLinejoin="round">
+          <path d="M12 3a6 6 0 0 0 9 9 9 9 0 1 1-9-9Z" />
+        </svg>
+      )}
+    </button>
+  );
+}