Lower the battery threshold to 30% and default the cachy-update prompt to yes

The prompt offering to silence cachy-update's own update notification now
defaults to yes: once updates install themselves that notification is nothing
but noise, so the common answer should be the one you get by pressing Enter.

It also accepts "j" now. The prompt is translated, so a German user reads
"[J/n]" and types the German letter - which the old check for "y" alone
silently read as a refusal.

res/config/cachy-auto-update.conf

@@ -31,7 +31,7 @@ UpdateInterval=1d
 
 # Never update while the battery is below this percentage. Ignored on mains
 # power and on machines without a battery.
-MinBatteryPercent=40
+MinBatteryPercent=30
 
 # Update only while plugged in. Stricter than MinBatteryPercent.
 RequireAC=no