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/arrow-left-BuPfyTxu.js

@@ -0,0 +1,6 @@
+import{c as e}from"./index-D72MDfqC.js";/**
+ * @license lucide-vue-next v1.0.0 - ISC
+ *
+ * This source code is licensed under the ISC license.
+ * See the LICENSE file in the root directory of this source tree.
+ */const t=e("arrow-left",[["path",{d:"m12 19-7-7 7-7",key:"1l729n"}],["path",{d:"M19 12H5",key:"x3x0zl"}]]);export{t as A};