fix: detect muted rooms with empty actions array
The mute detection was checking for `actions[0] === "dont_notify"` but
Cinny sets `actions: []` (empty array) when muting a room, which is
the correct behavior per Matrix spec where empty actions means no
notification.
This caused muted rooms to still show unread badges and contribute to
space badge counts.
Fixes the isMutedRule check to handle both:
- Empty actions array (current Matrix spec)
- "dont_notify" string (deprecated but may exist in older rules)
fd37dfe3f9