feat: switch AI provider to Claude Sonnet 4.6 + add shift+enter hint to chat inputs

- Default AI_PROVIDER changed from gemini to anthropic
- AI_MODEL and AI_MODEL_ANTHROPIC updated to claude-sonnet-4-6
- Added "Shift + Enter for a new line" hint below all chat textareas

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

backend/app/core/config.py

@@ -74,15 +74,15 @@ class Settings(BaseSettings):
 
     # AI Flow Builder
     ANTHROPIC_API_KEY: Optional[str] = None
-    AI_MODEL: str = "claude-haiku-4-5-20251001"
+    AI_MODEL: str = "claude-sonnet-4-6"
     AI_CONVERSATION_TTL_HOURS: int = 24
     AI_MAX_CALLS_PER_FLOW: int = 10
     AI_REQUEST_TIMEOUT_SECONDS: int = 45
     # AI Provider selection
-    AI_PROVIDER: str = "gemini"  # "gemini" or "anthropic"
+    AI_PROVIDER: str = "anthropic"  # "gemini" or "anthropic"
     GOOGLE_AI_API_KEY: Optional[str] = None
     AI_MODEL_GEMINI: str = "gemini-2.5-flash"
-    AI_MODEL_ANTHROPIC: str = "claude-haiku-4-5-20251001"
+    AI_MODEL_ANTHROPIC: str = "claude-sonnet-4-6"
 
     # MCP (Model Context Protocol) integrations
     ENABLE_MCP_MICROSOFT_LEARN: bool = True