[FIX] web: restore responsive mode for website only

Due to a recent change: https://github.com/odoo/odoo/commit/c3335a0db03b7ef5e98bddb7c15b143510d58c1e#diff-c7dab7f11ffa06829bbc6ac5c87a99a8L420
website wasn't responsive anymore.
This was not intended as we only wanted to remove the
responsive support for the backend, not the frontend.

closes odoo/odoo#40314

Signed-off-by: Adrien Dieudonné (adr) <adr@odoo.com>

addons/web/views/webclient_templates.xml

@@ -438,6 +438,9 @@
     </template>
 
     <template id="web.frontend_layout" name="Frontend Layout" inherit_id="web.layout" primary="True">
+        <xpath expr="//head/meta[last()]" position="after">
+            <meta name="viewport" content="width=device-width, initial-scale=1, user-scalable=no"/>
+        </xpath>
         <xpath expr="//head/link[last()]" position="after">
             <t t-call-assets="web.assets_common" t-js="false"/>
             <t t-call-assets="web.assets_frontend" t-js="false"/>