## Problem
Channels page shows 53K 'Unknown' messages — undecryptable GRP_TXT
packets with no content. Pure noise.
## Fix
- Backend: channels API filters out undecrypted messages by default
- `?includeEncrypted=true` param to include them
- Frontend: 'Show encrypted' toggle in channels sidebar
- Unknown channels grayed out with '(no key)' label
- Toggle persists in localStorage
Fixes#727
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Co-authored-by: you <you@example.com>