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

frontend/src/components/ai-chat/ChatInput.tsx

@@ -37,7 +37,8 @@ export function ChatInput({ onSend, disabled, placeholder = 'Type a message...'
   }
 
   return (
-    <div className="flex items-end gap-2 border-t border-border bg-card px-4 py-3">
+    <div className="border-t border-border bg-card px-4 py-3">
+      <div className="flex items-end gap-2">
       <textarea
         ref={textareaRef}
         value={value}
@@ -69,5 +70,7 @@ export function ChatInput({ onSend, disabled, placeholder = 'Type a message...'
         <Send className="h-4 w-4" />
       </button>
       </div>
+      <p className="text-[0.625rem] text-muted-foreground mt-1.5 px-1">Shift + Enter for a new line</p>
+    </div>
   )
 }

frontend/src/components/copilot/CopilotPanel.tsx

@@ -174,6 +174,7 @@ export function CopilotPanel({ isOpen, onClose, treeId, sessionId, currentNodeId
             <Send size={16} />
           </button>
         </div>
+        <p className="text-[0.625rem] text-muted-foreground mt-1.5 px-1">Shift + Enter for a new line</p>
       </div>
     </div>
   )

frontend/src/pages/AssistantChatPage.tsx

@@ -233,7 +233,8 @@ export default function AssistantChatPage() {
 
             {/* Input */}
             <div className="px-6 py-4 border-t shrink-0" style={{ borderColor: 'var(--glass-border)' }}>
-              <div className="flex items-end gap-3 max-w-3xl mx-auto">
+              <div className="max-w-3xl mx-auto">
+                <div className="flex items-end gap-3">
                   <textarea
                     ref={inputRef}
                     value={input}
@@ -267,6 +268,8 @@ export default function AssistantChatPage() {
                     )}
                   </div>
                 </div>
+                <p className="text-[0.625rem] text-muted-foreground mt-1.5 px-1">Shift + Enter for a new line</p>
+              </div>
             </div>
           </>
         ) : (