Replace fontsource with next/font/google and update react-zen.

- Switch from @fontsource/inter to next/font/google for font loading
- Update @umami/react-zen to v0.229.0
- Add type augmentation for react-zen missing children props
- Move global.css to app directory

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/types/react-zen.d.ts

@@ -0,0 +1,12 @@
+import { ReactNode } from 'react';
+import '@umami/react-zen';
+
+declare module '@umami/react-zen' {
+  interface SelectProps {
+    children?: ReactNode;
+  }
+
+  interface TooltipProps {
+    children?: ReactNode;
+  }
+}