feat: add channel/type icons to session list in sidebar

Show Discord, Telegram, WhatsApp, Signal, Slack brand icons,
clock icon for cron sessions, bot icon for sub-agents, and
globe icon for webchat sessions. Falls back to message bubble
for unknown channels.

Closes feedback #23

src/types/index.ts

@@ -24,6 +24,8 @@ export interface Session {
   contextTokens?: number;
   inputTokens?: number;
   outputTokens?: number;
+  channel?: string;
+  kind?: string;
 }
 
 export type ConnectionStatus = 'disconnected' | 'connecting' | 'connected';