Freshsales supported custom apps in the form of "sidebar apps" but did not support "full-page apps." A sidebar app is an app that appears as a widget in the sidebar of the Freshsales user interface, typically accessible from various pages such as Contacts, Deals, Accounts, etc. This sidebar app allows you to extend the functionality of Freshsales by adding custom actions or integrations related to the specific page where it appears.
On the other hand, a full-page app would mean an app that takes over the entire Freshsales interface or opens in a new browser tab, providing a more extensive user interface and functionality beyond what the sidebar app can offer.
The app manifest for Freshsales would indeed need to use "freshworks_crm" to work correctly, as opposed to using "freshsales". This might be specific to the way the platform is organized internally.
Since the information may have changed after my last update, I recommend checking Freshsales' official documentation or reaching out to their support team to get the most up-to-date information on app development and supported app types. They might have made updates or introduced new features that allow for different types of custom apps.