fix: async document processing in background threads, improve chat UX

- Fix knowledge_base_service calling async process_document synchronously
  in background threads — use asyncio.new_event_loop().run_until_complete()
  instead of bare call that returned unawaited coroutine
- Same fix for _update_document and reindex_document
- Replace ModeSelector with auto-detect: RAG mode when knowledge bases selected
- Convert regenerateMessage to streaming (was synchronous API call)
- Show "知识库检索模式" indicator above input when KB selected
- Remove loadSessions side-effect from streaming onComplete
- Fix chat page loading state to avoid flash
- Improve forum page spacing and sizing

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/services/knowledge_base_service.py

@@ -161,14 +161,16 @@ class KnowledgeBaseService:
             
             # 处理文档（向量化）- 在后台异步处理，不阻塞主流程
             try:
-                # 使用同步方法，但不等待完成（在后台处理）
                 import threading
+                import asyncio as _asyncio
                 def process_in_background():
                     try:
-                        self.document_service.process_document(document.id)
+                        loop = _asyncio.new_event_loop()
+                        loop.run_until_complete(self.document_service.process_document(document.id))
+                        loop.close()
                     except Exception as e:
                         print(f"后台处理文档 {document.id} 失败: {e}")
-                
+
                 thread = threading.Thread(target=process_in_background, daemon=True)
                 thread.start()
             except Exception as e:
@@ -198,12 +200,15 @@ class KnowledgeBaseService:
             # 重新处理文档 - 在后台异步处理
             try:
                 import threading
+                import asyncio as _asyncio
                 def process_in_background():
                     try:
-                        self.document_service.process_document(document.id)
+                        loop = _asyncio.new_event_loop()
+                        loop.run_until_complete(self.document_service.process_document(document.id))
+                        loop.close()
                     except Exception as e:
                         print(f"后台处理文档 {document.id} 失败: {e}")
-                
+
                 thread = threading.Thread(target=process_in_background, daemon=True)
                 thread.start()
             except Exception as e:
@@ -340,7 +345,10 @@ class KnowledgeBaseService:
             self.db.query(DocumentChunk).filter(DocumentChunk.document_id == document_id).delete()
             
             # 重新处理文档
-            success = self.document_service.process_document(document_id)
+            import asyncio as _asyncio
+            success = _asyncio.get_event_loop().run_until_complete(
+                self.document_service.process_document(document_id)
+            )
             
             if success:
                 return {"success": True, "message": "文档重新索引成功"}