feat(web): add merge hint to CLI and MCP user credentials dialogs

Both dialogs now show an amber callout explaining that chat users
(Telegram, Discord, etc.) must be merged into a tenant user via
Contacts page before they can have per-user credentials.
i18n: en/vi/zh for both cli-credentials and mcp namespaces.

ui/web/src/i18n/locales/en/cli-credentials.json

@@ -81,6 +81,7 @@
     "saveFailed": "Failed to save user credentials",
     "deleted": "User credentials deleted",
     "deleteFailed": "Failed to delete user credentials",
-    "envRequired": "At least one environment variable is required"
+    "envRequired": "At least one environment variable is required",
+    "mergeHint": "Chat users (Telegram, Discord, etc.) must be merged into a tenant user first via Contacts page before they can have per-user credentials."
   }
 }

ui/web/src/i18n/locales/en/mcp.json

@@ -127,6 +127,7 @@
     "addHeader": "Add Header",
     "addEnv": "Add Variable",
     "saveFailed": "Failed to save credentials",
-    "deleteFailed": "Failed to delete credentials"
+    "deleteFailed": "Failed to delete credentials",
+    "mergeHint": "Chat users (Telegram, Discord, etc.) must be merged into a tenant user first via Contacts page before they can have per-user credentials."
   }
 }

ui/web/src/i18n/locales/vi/cli-credentials.json

@@ -81,6 +81,7 @@
     "saveFailed": "Không thể lưu thông tin người dùng",
     "deleted": "Đã xóa thông tin người dùng",
     "deleteFailed": "Không thể xóa thông tin người dùng",
-    "envRequired": "Cần ít nhất một biến môi trường"
+    "envRequired": "Cần ít nhất một biến môi trường",
+    "mergeHint": "Người dùng chat (Telegram, Discord...) cần được gộp vào tenant user qua trang Contacts trước khi có thể thiết lập credentials riêng."
   }
 }

ui/web/src/i18n/locales/vi/mcp.json

@@ -127,6 +127,7 @@
     "addHeader": "Thêm Header",
     "addEnv": "Thêm biến",
     "saveFailed": "Lưu credentials thất bại",
-    "deleteFailed": "Xóa credentials thất bại"
+    "deleteFailed": "Xóa credentials thất bại",
+    "mergeHint": "Người dùng chat (Telegram, Discord...) cần được gộp vào tenant user qua trang Contacts trước khi có thể thiết lập credentials riêng."
   }
 }

ui/web/src/i18n/locales/zh/cli-credentials.json

@@ -81,6 +81,7 @@
     "saveFailed": "保存用户凭证失败",
     "deleted": "用户凭证已删除",
     "deleteFailed": "删除用户凭证失败",
-    "envRequired": "至少需要一个环境变量"
+    "envRequired": "至少需要一个环境变量",
+    "mergeHint": "聊天用户（Telegram、Discord 等）需要先通过联系人页面合并为租户用户，才能设置独立凭证。"
   }
 }

ui/web/src/i18n/locales/zh/mcp.json

@@ -127,6 +127,7 @@
     "addHeader": "添加 Header",
     "addEnv": "添加变量",
     "saveFailed": "保存凭据失败",
-    "deleteFailed": "删除凭据失败"
+    "deleteFailed": "删除凭据失败",
+    "mergeHint": "聊天用户（Telegram、Discord 等）需要先通过联系人页面合并为租户用户，才能设置独立凭证。"
   }
 }

ui/web/src/pages/cli-credentials/cli-user-credentials-dialog.tsx

@@ -240,6 +240,7 @@ export function CLIUserCredentialsDialog({ open, onOpenChange, binary }: CLIUser
                   source="tenant_user"
                   allowCustom
                 />
+                <p className="text-xs text-amber-600 dark:text-amber-400 bg-amber-50 dark:bg-amber-950/30 rounded-md px-2.5 py-1.5 border border-amber-200 dark:border-amber-800">{t("userCredentials.mergeHint")}</p>
               </div>
 
               <div className="flex flex-col gap-1.5">

ui/web/src/pages/mcp/mcp-user-credentials-dialog.tsx

@@ -147,7 +147,8 @@ export function MCPUserCredentialsDialog({
                   onChange={setSelectedUserId}
                   placeholder={t("userCredentials.selectUser")}
                   source="tenant_user"
-                  />
+                />
+                <p className="text-xs text-amber-600 dark:text-amber-400 bg-amber-50 dark:bg-amber-950/30 rounded-md px-2.5 py-1.5 border border-amber-200 dark:border-amber-800">{t("userCredentials.mergeHint")}</p>
               </div>
             )}