fix(ui): show specific error reason when expert chat is not ready

When ensureChatAvailable fails for expert mode, the error message
previously used chatStatusMessage which shows "聊天服务已就绪" when
the runtime/gateway are healthy — misleading the user when the real
issue is project-level readiness (projectReady=false or missing
currentProjectId).

Now the error message identifies the actual failure:
- 项目工作区未就绪 when projectReady is false
- 未选择项目 when currentProjectId is missing
- 本地运行时未就绪 when runtime is not running
- 网关未连接 when gateway is not connected
- Falls back to the original chatStatusMessage for uncovered cases
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/ui/src/features/chat/useChatStreamingController.ts

@@ -495,7 +495,31 @@ export function useChatStreamingController(deps: UseChatStreamingControllerDeps)
       return confirmedWorkspace
     }
 
-    throw new Error(confirmedWorkspace.chatStatusMessage ?? ui.chatNotReadyError)
+    // Build a specific error message identifying the actual failure reason,
+    // rather than blindly using chatStatusMessage which may say "chat ready"
+    // when the real problem is project-level readiness.
+    const missingParts: string[] = []
+    if (confirmedViewMode === "experts") {
+      if (confirmedWorkspace.chatReady && !confirmedWorkspace.projectReady) {
+        missingParts.push("项目工作区未就绪")
+      }
+      if (!confirmedWorkspace.currentProjectId) {
+        missingParts.push("未选择项目")
+      }
+    }
+    if (confirmedWorkspace.chatReady && !confirmedCanExchange) {
+      const runtimeReady = confirmedRuntime?.activeMode === "external-gateway" || confirmedRuntime?.processState === "running"
+      if (!runtimeReady) {
+        missingParts.push("本地运行时未就绪")
+      } else if (confirmedGateway?.state !== "connected") {
+        missingParts.push("网关未连接")
+      }
+    }
+    throw new Error(
+      missingParts.length > 0
+        ? `${missingParts.join("，")}，请稍后重试`
+        : (confirmedWorkspace.chatStatusMessage ?? ui.chatNotReadyError)
+    )
   }, [appendTrace, desktopApi.gateway, desktopApi.runtime, desktopApi.workspace, setGatewayStatus, setRuntimeStatus, setWorkspace, ui.bindFirstError, ui.chatNotReadyError, ui.checkingChat, ui.checkingChatDetail, ui.connectingGateway, ui.connectingGatewayDetail, ui.startingRuntime, ui.startingRuntimeDetail, updateAssistantStatus])
 
   const submitPrompt = useCallback(async ({