ux(flow): remove facebook/line channel everywhere (scenario chosen at chat); clarify 'how to start chatting'

- Removed channel + initiation displays: Training card, Personas card, GroupEdit card,
  Chat header, and the channel select in PersonaForm (scenario now determines who opens).
- Clarified the start-training flow: GroupEdit guide now says go to the Training tab ->
  pick product -> pick a customer -> Chat -> choose scenario -> talk; Personas guide
  spells out the Chat + scenario step.
Rebuilt dist.

frontend/dist/assets/GroupEdit-Bujf8Tz9.css

@@ -0,0 +1 @@
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