fix: correct env var add form action path

The 'Add' environment variable form in app_detail.html was posting to
/apps/{id}/env, but the route is registered as /apps/{id}/env-vars.
This path mismatch caused a 404 when trying to add environment variables
from the app detail page.

Fix the form action to use the correct /apps/{id}/env-vars path,
matching the route defined in routes.go.

closes #156

templates/app_detail.html

@@ -146,7 +146,7 @@
             </table>
         </div>
         {{end}}
-        <form method="POST" action="/apps/{{.App.ID}}/env" class="flex flex-col sm:flex-row gap-2">
+        <form method="POST" action="/apps/{{.App.ID}}/env-vars" class="flex flex-col sm:flex-row gap-2">
                 {{ .CSRFField }}
             <input type="text" name="key" placeholder="KEY" required class="input flex-1 font-mono text-sm">
             <input type="text" name="value" placeholder="value" required class="input flex-1 font-mono text-sm">