feat: add theme toggle functionality and update localization files

- Implemented a theme toggle feature allowing users to switch between light and dark modes.
- Added a new ThemeToggle component for the theme switching button.
- Updated theme CSS to support dark mode variables and system preference detection.
- Enhanced localization files to include translations for the "toggleTheme" key across multiple languages.

src/components/themeToggle.tsx

@@ -0,0 +1,55 @@
+import { type SupportedLocale, type Translator, createTranslator, defaultLocale } from "../i18n/index";
+
+export const ThemeToggle = ({
+  locale = defaultLocale,
+  t = createTranslator(defaultLocale),
+}: {
+  locale?: SupportedLocale;
+  t?: Translator;
+}) => {
+  return (
+    <button
+      type="button"
+      id="theme-toggle"
+      class={`
+        flex items-center gap-1 text-accent-600 transition-all
+        hover:text-accent-500
+      `}
+      aria-label={t("nav", "toggleTheme")}
+      title={t("nav", "toggleTheme")}
+    >
+      {/* Sun icon (shown in dark mode) */}
+      <svg
+        id="theme-icon-light"
+        xmlns="http://www.w3.org/2000/svg"
+        fill="none"
+        viewBox="0 0 24 24"
+        stroke-width="1.5"
+        stroke="currentColor"
+        class="hidden h-6 w-6"
+      >
+        <path
+          stroke-linecap="round"
+          stroke-linejoin="round"
+          d="M12 3v2.25m6.364.386l-1.591 1.591M21 12h-2.25m-.386 6.364l-1.591-1.591M12 18.75V21m-4.773-4.227l-1.591 1.591M5.25 12H3m4.227-4.773L5.636 5.636M15.75 12a3.75 3.75 0 11-7.5 0 3.75 3.75 0 017.5 0z"
+        />
+      </svg>
+      {/* Moon icon (shown in light mode) */}
+      <svg
+        id="theme-icon-dark"
+        xmlns="http://www.w3.org/2000/svg"
+        fill="none"
+        viewBox="0 0 24 24"
+        stroke-width="1.5"
+        stroke="currentColor"
+        class="hidden h-6 w-6"
+      >
+        <path
+          stroke-linecap="round"
+          stroke-linejoin="round"
+          d="M21.752 15.002A9.718 9.718 0 0118 15.75c-5.385 0-9.75-4.365-9.75-9.75 0-1.33.266-2.597.748-3.752A9.753 9.753 0 003 11.25C3 16.635 7.365 21 12.75 21a9.753 9.753 0 009.002-5.998z"
+        />
+      </svg>
+    </button>
+  );
+};