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/src/App.tsx

@@ -81,7 +81,7 @@ function Footer({ appName = 'DearDiary' }: { appName?: string }) {
   const { resolvedTheme } = useTheme();
   return (
     <footer className={`py-6 text-center text-sm text-slate-500 ${resolvedTheme === 'dark' ? 'border-t border-slate-800' : 'border-t border-slate-200'}`}>
-      <p>{appName} v{packageJson.version} — Self-hosted AI-powered journaling · <a href={import.meta.env.VITE_GIT_URL || 'https://git.kropa.tech/lotherk/deardiary'} className="hover:text-purple-400 transition">Git Repository</a> · MIT License · © 2026 Konrad Lother</p>
+      <p>{appName} v{packageJson.version} · <a href={import.meta.env.VITE_GIT_URL || 'https://git.kropa.tech/lotherk/deardiary'} target="_blank" rel="noopener noreferrer" className="hover:text-purple-400 transition">Git Repository</a> · <a href={`${import.meta.env.VITE_GIT_URL || 'https://git.kropa.tech/lotherk/deardiary'}/src/branch/main/LICENSE'} target="_blank" rel="noopener noreferrer" className="hover:text-purple-400 transition">License</a></p>
     </footer>
   );
 }

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'),
     },
   };
 });