Profile directory and app name change after update to 1.2.1 causes logout and new security session

### Bug summary

After updating to Fastmail 1.2.1 from Flathub (`com.fastmail.Fastmail`, commit `cafd559717c06d24e53010931b8b5bbfaf37a39a3043059378c51f953cd2d990`, the following changes were observed:

**1. New user profile directory is created:**
- Previously, the config/profile directory used by the app was:
`~/.var/app/com.fastmail.Fastmail/config/Fastmail`
- After the update, a new directory is created:
`~/.var/app/com.fastmail.Fastmail/config/com.fastmail.Fastmail`

**2. Forced re-login:**
- Users are forced to log in again, since the app no longer sees the previous profile/session data stored under the prior directory.

**3. GNOME display name change:**
- GNOME overview and Software now displays the Flatpak app-id `com.fastmail.Fastmail` instead of the proper name “Fastmail” in the app grid.

**4. Fastmail security dashboard shows new/unknown app entry:**
- Logging in again as required above produces an additional "Unknown app" entry in Fastmail's list of connected sessions rather than recognizing it as the previous “Flatpak app”.

### Expected behavior
- Profile/config directory should remain stable across updates if possible, to avoid lost sessions and user confusion
- Gnome app grid should display the intended app name (not the app-id) if metadata allows

### Additional notes
- This appears may be caused by a change in the upstream AppImage (Electron application name)? Perhaps passing a profile directory in `fastmail.sh` could help though I don't know if that would fix all observed changes.