feat: migrate KnowledgeView mini-chat to ChatPanel + enable note picker everywhere

- KnowledgeView: replace custom mini-chat (voice, PTT, manual scroll, message
  rendering) with ChatPanel variant="full"; gains RAG scope chip, TTS listen
  mode, volume control, and note picker automatically (-218 lines)
- ChatInputBar: remove briefingMode guard on note picker so attach/search works
  in briefing, workspace, widget, and knowledge chat surfaces

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ChatInputBar.vue

@@ -160,7 +160,7 @@ defineExpose({ focus, prefill })
 
     <div class="input-row">
       <!-- Note picker -->
-      <div v-if="!briefingMode" class="note-picker-wrapper">
+      <div class="note-picker-wrapper">
         <button
           class="btn-icon"
           @click="toggleNotePicker"