fix: default theme to light instead of following system preference

New users on dark-mode systems were seeing dark theme on first visit.
The default is now explicitly light, matching the API's DEFAULT_THEME.
Users can still switch to dark or system in settings.

apps/web/src/stores/theme-store.ts

@@ -17,8 +17,8 @@ function getSystemTheme(): "light" | "dark" {
 export const useThemeStore = create<ThemeStore>()(
   persist(
     (set) => ({
-      theme: "system",
-      resolvedTheme: getSystemTheme(),
+      theme: "light" as Theme,
+      resolvedTheme: "light" as const,
       setTheme: (theme) => {
         const resolved = theme === "system" ? getSystemTheme() : theme;
         document.documentElement.classList.toggle("dark", resolved === "dark");