style: 全局 UI 主题皮肤重构与样式模块化

引入 Element Plus 主题定制和主题皮肤 composable，将全局
样式拆分为组件级独立 CSS 文件（侧边栏、顶栏、工作台等），
统一色彩变量和间距规范，重构所有视图和组件样式以适配新
主题系统，优化图表和知识图谱组件视觉表现，提取审计和差
旅报销相关子组件。

web/src/views/LlmSettingsPanel.vue

@@ -28,9 +28,12 @@
       <div class="form-grid">
         <label class="field">
           <span><em>*</em> 供应商</span>
-          <select v-model="llmForm.mainProvider" @change="applyProviderPreset('main')">
-            <option v-for="option in providerOptions" :key="option" :value="option">{{ option }}</option>
-          </select>
+          <EnterpriseSelect
+            v-model="llmForm.mainProvider"
+            :options="providerOptions"
+            placeholder="选择供应商"
+            @change="applyProviderPreset('main')"
+          />
         </label>
 
         <label class="field">
@@ -101,9 +104,12 @@
       <div class="form-grid">
         <label class="field">
           <span><em>*</em> 供应商</span>
-          <select v-model="llmForm.backupProvider" @change="applyProviderPreset('backup')">
-            <option v-for="option in providerOptions" :key="option" :value="option">{{ option }}</option>
-          </select>
+          <EnterpriseSelect
+            v-model="llmForm.backupProvider"
+            :options="providerOptions"
+            placeholder="选择供应商"
+            @change="applyProviderPreset('backup')"
+          />
         </label>
 
         <label class="field">
@@ -174,9 +180,12 @@
       <div class="form-grid">
         <label class="field">
           <span><em>*</em> 供应商</span>
-          <select v-model="llmForm.embeddingProvider" @change="applyProviderPreset('embedding')">
-            <option v-for="option in providerOptions" :key="option" :value="option">{{ option }}</option>
-          </select>
+          <EnterpriseSelect
+            v-model="llmForm.embeddingProvider"
+            :options="providerOptions"
+            placeholder="选择供应商"
+            @change="applyProviderPreset('embedding')"
+          />
         </label>
 
         <label class="field">
@@ -251,9 +260,12 @@
       <div class="form-grid">
         <label class="field">
           <span><em>*</em> 供应商</span>
-          <select v-model="llmForm.rerankerProvider" @change="applyProviderPreset('reranker')">
-            <option v-for="option in providerOptions" :key="option" :value="option">{{ option }}</option>
-          </select>
+          <EnterpriseSelect
+            v-model="llmForm.rerankerProvider"
+            :options="providerOptions"
+            placeholder="选择供应商"
+            @change="applyProviderPreset('reranker')"
+          />
         </label>
 
         <label class="field">
@@ -306,4 +318,3 @@
 
 <style scoped src="../assets/styles/views/settings-view-form.css"></style>
 <style scoped src="../assets/styles/views/settings-view.css"></style>
-