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.

README.md

@@ -74,7 +74,7 @@ deliberately.
 
 A run is postponed — and retried an hour later — when:
 
-- the battery is below 40 % (ignored on mains power; desktops without a battery
+- the battery is below 30 % (ignored on mains power; desktops without a battery
   are never affected),
 - a game is running: GameMode, a known game process, or anything holding a
   blocking idle inhibitor,