Default to all-but-one core for encoding

Using every core for a parallel batch starves the compositor and makes
the GUI show "not responding". Default multicore to cores-1 so the
desktop keeps headroom; the user can still raise it in Preferences.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/devedeng/configuration_data.py

@@ -102,7 +102,9 @@ class configuration(GObject.GObject):
 
         self.PAL = True
         self.tmp_folder = "/var/tmp"
-        self.multicore = self.cores
+        # default to all-but-one core so a big parallel batch leaves the
+        # desktop responsive; the user can raise this in Preferences
+        self.multicore = max(1, self.cores - 1)
         self.final_folder = os.environ.get("HOME")
         self.sub_language = None
         self.sub_codepage = None