🤖 fix(storybook): change default theme to dark

Changes the default Storybook theme from light to dark, affecting:
- The default 'Latest' view when no mode is explicitly set
- Server-side rendering (when window is undefined)
- Stories when no persisted theme exists in localStorage

Chromatic's explicit mode variants (dark/light) are unaffected.

Author: ethanndickson

.storybook/preview.tsx

@@ -41,12 +41,12 @@ const applyStorybookTheme = (mode: ThemeMode) => {
 
 const syncStorybookTheme = (mode?: ThemeMode): ThemeMode => {
   if (typeof window === "undefined") {
-    return mode ?? "light"; // Keep default aligned with baseline to avoid Chromatic diffs
+    return mode ?? "dark"; // Default to dark theme
   }
 
   const stored = window.localStorage.getItem(UI_THEME_KEY);
   const persisted = stored ? (JSON.parse(stored) as ThemeMode) : undefined;
-  const resolved = mode ?? persisted ?? "light"; // Default to light for Latest when unset
+  const resolved = mode ?? persisted ?? "dark"; // Default to dark for Latest when unset
   applyStorybookTheme(resolved);
   return resolved;
 };