feat: add website link to app footer, Join Alpha opens in new tab

Fixes #2 - Add website link to app footer
- Add VITE_WEBSITE_URL build arg for configurable website URL
- Add Git Repository and License links to app footer
- Update Join Free Alpha links to open in new tab (target=_blank)

frontend/vite.config.ts

@@ -17,6 +17,7 @@ export default defineConfig(({ mode }) => {
     },
     define: {
       'import.meta.env.VITE_GIT_URL': JSON.stringify(env.VITE_GIT_URL || 'https://git.kropa.tech/lotherk/deardiary'),
+      'import.meta.env.VITE_WEBSITE_URL': JSON.stringify(env.VITE_WEBSITE_URL || 'https://deardiary.lother.io'),
     },
   };
 });