feat(s2s-text): dedicated text-mode prompt + Markdown rendering

Architecture fix: voice and text mode now have completely separate prompts.

Backend:
- VoiceAssistantProfileSupport.buildTextSystemRole: dedicated text-mode system
  role that inherits all business rules (identity, KB-first, sensitive topics,
  sales guidance, personal info) but removes voice-specific constraints (short
  sentences, colloquial, single-line conclusion).
- DEFAULT_TEXT_SPEAKING_STYLE: text-specific style demanding detailed,
  structured, Markdown-formatted answers with complete information.
- VoiceGatewayService.handleStart: switch between voice/text system role and
  speaking style based on state.textMode.
- VoiceGatewayService.buildStartSessionPayload: preserve Markdown in text mode
  (voice mode still strips asterisks/backticks via normalizeTextForSpeech to
  avoid TTS pronouncing format chars).

Frontend:
- Added react-markdown@9 + remark-gfm@4 dependencies.
- ChatPanel renders assistant messages (non-voice) with ReactMarkdown:
  headings, lists (ul/ol), bold, italic, inline/block code, tables, blockquote,
  links, horizontal rules — all styled with Tailwind classes matching the dark
  theme.
- User messages and voice-handoff messages remain plain text.

Verification: mvn test VoiceGatewaySmokeTest 20/20 pass, vite build succeeds.

test2/client/package.json

@@ -13,7 +13,9 @@
     "axios": "^1.6.2",
     "lucide-react": "^0.344.0",
     "react": "^18.2.0",
-    "react-dom": "^18.2.0"
+    "react-dom": "^18.2.0",
+    "react-markdown": "^9.1.0",
+    "remark-gfm": "^4.0.1"
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.0.0",