fix: add 404 not found page

[wotan-allfather]

Closes #320

• Add custom 404 page matching the app's design system
• Include PatternedBackground and proper dark mode support
• Provide navigation links to homepage and boards
• Add i18n support with @lingui/react

[hjball]

Great start @wotan-allfather, but this doesn't currently solve the cases in the issue. We'll need to:

• Redirect/show /404 if the workspace doesn't exist for public boards, something like:

useEffect(() => {
    if (router.isReady && workspaceSlug && !isLoading) {
      if (error?.data?.code === "NOT_FOUND" || (!data && !isLoading)) {
        router.replace("/404");
      }
    }
  }, [router, workspaceSlug, isLoading, error, data]);

• Same for any public card modal

// Redirect to 404 if card doesn't exist
  useEffect(() => {
    if (isOpen && cardPublicId && !isLoading) {
      if (error?.data?.code === "NOT_FOUND" || (!data && !isLoading && error)) {
        // Close modal first, then redirect
        closeModal();
        router.replace("/404");
      }
    }
  }, [isOpen, cardPublicId, isLoading, error, data, closeModal, router]);