refactor: 重构 Plan 页面代码结构

- 抽取 usePlan composable 逻辑
- 分离 plan.css 样式文件
- 简化 Plan.vue 组件代码

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/views/plan/plan.css

@@ -0,0 +1,92 @@
+.plan-page {
+  background-color: #0f1419;
+}
+
+.search-input {
+  background-color: #1f2937;
+  border: 1px solid #374151;
+  border-radius: 8px;
+  padding: 10px 12px 10px 36px;
+  color: white;
+  font-size: 14px;
+  outline: none;
+  transition: border-color 0.2s;
+}
+
+.search-input:focus {
+  border-color: #f97316;
+}
+
+.search-input::placeholder {
+  color: #6b7280;
+}
+
+.table-row {
+  border-top: 1px solid #2a2a3a;
+  transition: background-color 0.2s;
+}
+
+.table-row:hover {
+  background-color: rgba(255, 255, 255, 0.02);
+}
+
+.task-tag {
+  background-color: #374151;
+  color: #d1d5db;
+  font-size: 11px;
+  padding: 2px 6px;
+  border-radius: 4px;
+}
+
+.ml-13 {
+  margin-left: 3.25rem;
+}
+
+.btn-icon {
+  padding: 6px;
+  border-radius: 6px;
+  transition: all 0.2s;
+}
+
+.btn-icon:hover {
+  background-color: rgba(255, 255, 255, 0.1);
+}
+
+.empty-box {
+  min-height: 300px;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+}
+
+.empty-icon {
+  width: 80px;
+  height: 80px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  background: linear-gradient(135deg, #1f2937, #111827);
+  border-radius: 20px;
+  margin-bottom: 16px;
+  color: #6b7280;
+}
+
+.empty-text {
+  font-size: 16px;
+  font-weight: 500;
+  color: white;
+  margin-bottom: 4px;
+}
+
+.empty-tip {
+  font-size: 14px;
+  color: #6b7280;
+}
+
+.line-clamp-2 {
+  display: -webkit-box;
+  -webkit-line-clamp: 2;
+  -webkit-box-orient: vertical;
+  overflow: hidden;
+}