feat(文件预览): 添加对PDF、Word和Excel文件的预览支持

添加新的文件预览组件，支持PDF、Word(docx)、Excel(xlsx/xls)文件类型预览
重构图片预览为独立组件，并添加相关依赖包

klp-ui/src/components/FilePreview/preview/pdf/index.vue

@@ -0,0 +1,34 @@
+<template>
+  <div class="preview-pdf">
+    <iframe
+      :src="src"
+      class="preview-pdf-content"
+      frameborder="0"
+    />
+  </div>
+</template>
+
+<script>
+export default {
+  name: "PdfPreview",
+  props: {
+    src: {
+      type: String,
+      required: true,
+      description: "PDF文件的URL或路径"
+    }
+  }
+};
+</script>
+
+<style scoped>
+.preview-pdf {
+  width: 100%;
+  height: 70vh;
+}
+
+.preview-pdf-content {
+  width: 100%;
+  height: 100%;
+}
+</style>