feat: add channels directory with 44 channel adapter definitions (#21)

* feat: add channels directory with 44 channel adapter definitions

Add TOML definitions for all 44 channel adapters supported by LibreFang,
matching the adapters in librefang-channels crate source code.

Each channel file includes: id, name, description, category, icon,
protocol, and metadata (url, docs).

Categories: messaging, social, enterprise, developer, iot, email.

Also adds channels template at templates/channel.toml.

* fix: format templates/channel.toml with taplo

channels/threema.toml

@@ -0,0 +1,10 @@
+id = "threema"
+name = "Threema"
+description = "Threema Gateway adapter for secure messaging via the Threema platform"
+category = "messaging"
+icon = "🔏"
+protocol = "rest-api"
+
+[metadata]
+url = "https://threema.ch"
+docs = "https://gateway.threema.ch/en/developer/api"