fix(service): start with the graphical session, not default.target (#24)

default.target is reached before the desktop exports DISPLAY/WAYLAND_DISPLAY
to the user manager, so After=graphical-session.target never applied. The
daemon's first keyring call D-Bus-activated the Secret Service without a
display; on KDE ksecretd aborted, the first start failed, and every login
left a coredump.

Install into graphical-session.target instead, and use reenable when the
front-ends enable the unit so existing default.target.wants links are
replaced rather than kept alongside the new one.

Fixes #23

docs/INSTALL.md

@@ -215,6 +215,14 @@ starts; to switch at once:
 systemctl --user restart proton-drive.service
 ```
 
+Coming from 2.8.0 or older, re-enable the service once. Those releases hooked it into
+`default.target`, which starts it before the desktop session is ready; on KDE that crashes
+`ksecretd` at every login. Signing in again or toggling "Start on login" does this as well:
+
+```bash
+systemctl --user reenable proton-drive.service
+```
+
 Database migrations run automatically and only move forward. A database written by a newer
 release refuses to open in an older one, so downgrading means clearing the local database (see
 [Troubleshooting](TROUBLESHOOTING.md#start-over-with-a-clean-local-state)).