fix(i18n): replace fragment translation keys with placeholder-based complete sentences

- Use {gravatar} placeholder in profile.avatar_upload_hint for proper i18n
- Use {settings_link} placeholder in subscription.single_user_body
- Use {api_tokens_link} placeholder in integrations.webhook_step1
- Add optional chaining in common.js for window.__i18n safety

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/templates/subscription.html

@@ -23,8 +23,8 @@
     <div>
       <p class="font-semibold text-blue-800">{{ _("subscription.single_user_title") }}</p>
       <p class="text-blue-700 text-sm mt-1">
-        {{ _("subscription.single_user_body_before_link") }}
-        <a href="/settings" class="underline hover:text-blue-900">{{ _("nav.settings") }}</a>{{ _("subscription.single_user_body_after_link") }}
+        {% set settings_link %}<a href="/settings" class="underline hover:text-blue-900">{{ _("nav.settings") }}</a>{% endset %}
+        {{ _("subscription.single_user_body", settings_link=settings_link) | safe }}
       </p>
     </div>
   </div>