feat(v3): Web 界面视觉升级（全局设计令牌 + 深色侧边栏壳 + 页面细节打磨）

- style.css 重写为设计系统：色板/圆角/阴影/统一滚动条/焦点环/卡片质感，main.ts 引入
- App.vue：深色渐变侧边栏替换顶部条（品牌区/图标导航/激活高亮/网关状态脚注）
- 六个视图页面背景令牌化，卡片统一阴影；index.html 标题与语言修正
- 功能零改动，纯视觉层

webapp/src/views/AgentView.vue

@@ -352,7 +352,7 @@ onMounted(async () => {
 .agent-view {
   display: flex;
   height: 100%;
-  background: #f9fafb;
+  background: var(--c-bg);
 }
 
 /* 左侧工作区 */
@@ -377,7 +377,7 @@ onMounted(async () => {
   padding: 6px 14px;
   font-size: 11px;
   color: #6b7280;
-  background: #f9fafb;
+  background: var(--c-bg);
   border-bottom: 1px solid #f3f4f6;
   white-space: nowrap;
   overflow: hidden;
@@ -411,7 +411,7 @@ onMounted(async () => {
   justify-content: space-between;
   align-items: center;
   padding: 8px 14px;
-  background: #f9fafb;
+  background: var(--c-bg);
   font-size: 12px;
   font-weight: 600;
   color: #374151;
@@ -623,7 +623,7 @@ onMounted(async () => {
 .crumb {
   font-size: 12px;
   color: #6b7280;
-  background: #f9fafb;
+  background: var(--c-bg);
   border: 1px solid #f3f4f6;
   border-radius: 6px;
   padding: 6px 10px;
@@ -666,7 +666,7 @@ onMounted(async () => {
 .recent-label { font-size: 12px; color: #9ca3af; }
 .recent-chip {
   border: 1px solid #e5e7eb;
-  background: #f9fafb;
+  background: var(--c-bg);
   border-radius: 12px;
   padding: 2px 10px;
   font-size: 12px;

webapp/src/views/ChatView.vue

@@ -178,7 +178,7 @@ async function handleSend() {
 .sidebar {
   width: 240px;
   border-right: 1px solid #e5e7eb;
-  background: #f9fafb;
+  background: var(--c-bg);
   display: flex;
   flex-direction: column;
   overflow: hidden;

webapp/src/views/CollaborationView.vue

@@ -245,7 +245,7 @@ function getDecision(issueId: string) {
 .collab-sidebar {
   width: 260px;
   border-right: 1px solid #e5e7eb;
-  background: #f9fafb;
+  background: var(--c-bg);
   display: flex;
   flex-direction: column;
   overflow: hidden;

webapp/src/views/MetricsView.vue

@@ -194,7 +194,7 @@ onMounted(load)
 .review-rate { font-size: 13px; color: #6b7280; margin: 0; }
 
 .raw-json {
-  background: #f9fafb;
+  background: var(--c-bg);
   border: 1px solid #e5e7eb;
   border-radius: 8px;
 }

webapp/src/views/ReviewView.vue

@@ -162,7 +162,7 @@ button.active { background: #2563eb; color: #fff; border-color: #2563eb; }
 }
 .query-block pre, .response-block pre {
   margin: 4px 0 0;
-  background: #f9fafb;
+  background: var(--c-bg);
   border: 1px solid #e5e7eb;
   border-radius: 4px;
   padding: 6px 10px;

webapp/src/views/SettingsView.vue

@@ -960,7 +960,7 @@ onUnmounted(() => { _dlSSE?.close() })
   height: 100%;
   overflow-y: auto;
   padding: 24px;
-  background: #f9fafb;
+  background: var(--c-bg);
   box-sizing: border-box;
   /* 覆盖 router-view 注入的 .content 的 display:flex：flex 列布局会把
      overflow:hidden 的子项压缩截断而非撑出滚动条，导致页面无法滚动 */
@@ -1063,7 +1063,7 @@ onUnmounted(() => { _dlSSE?.close() })
   justify-content: space-between;
   align-items: center;
   padding: 8px 12px;
-  background: #f9fafb;
+  background: var(--c-bg);
   border-bottom: 1px solid #f3f4f6;
   font-size: 13px;
   font-weight: 500;
@@ -1273,7 +1273,7 @@ onUnmounted(() => { _dlSSE?.close() })
 .btn-inline { font-size: 12px; }
 
 .btn-ping {
-  background: #f9fafb;
+  background: var(--c-bg);
   border: 1px solid #d1d5db;
   border-radius: 6px;
   padding: 6px 10px;