perf: 性能优化与代码质量改进

- 添加4个数据库索引提升查询性能
- 启用ESLint并配置React支持
- 修复ESLint错误（未使用的导入和状态）
- 简化Toast组件UI样式
- 移除重复文件(server.js.backup)
- 清理.gitignore中的.next目录

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

frontend/styles/globals.css

@@ -1755,53 +1755,53 @@ textarea:focus-visible {
 
 .toast-stack {
   position: fixed;
-  left: 50%;
+  left: 0;
+  right: 0;
   bottom: calc(var(--bottom-nav-height) + var(--safe-bottom) + 14px);
-  transform: translateX(-50%);
-  width: min(calc(100% - 28px), 480px);
-  display: grid;
-  gap: 8px;
+  display: flex;
+  flex-direction: column;
+  align-items: center;
   z-index: 90;
+  pointer-events: none;
 }
 
 .toast {
-  display: grid;
-  grid-template-columns: auto 1fr auto;
-  align-items: center;
-  gap: 10px;
-  padding: 12px 13px;
-  border-radius: 15px;
-  background: var(--surface-elevated);
-  color: var(--text-on-dark);
-  box-shadow: 0 16px 34px rgba(15, 23, 42, 0.14);
-  backdrop-filter: blur(18px);
-}
-
-.toast__icon,
-.toast__close {
-  color: inherit;
+  padding: 10px 24px;
+  border-radius: 22px;
+  background: rgba(0, 0, 0, 0.75);
+  color: #fff;
+  font-size: 14px;
+  font-weight: 500;
+  text-align: center;
+  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.15);
+  pointer-events: auto;
+  width: fit-content;
+  max-width: 90%;
+  overflow: visible;
 }
 
 .toast__message {
-  line-height: 1.42;
-}
-
-.toast__close.icon-button {
-  width: 30px;
-  height: 30px;
-  border-radius: 10px;
+  display: inline;
+  font-size: 14px;
+  font-weight: 500;
+  color: #fff;
+  text-align: center;
+  line-height: 1.4;
+  white-space: normal;
+  unicode-bidi: bidi-override;
+  direction: ltr;
 }
 
 .toast--success {
-  background: linear-gradient(135deg, rgba(5, 150, 105, 0.96), rgba(6, 182, 212, 0.9));
+  background: rgba(0, 0, 0, 0.75);
 }
 
 .toast--error {
-  background: linear-gradient(135deg, rgba(220, 38, 38, 0.96), rgba(244, 63, 94, 0.92));
+  background: rgba(220, 38, 38, 0.9);
 }
 
 .toast--info {
-  background: linear-gradient(135deg, rgba(15, 23, 42, 0.94), rgba(51, 65, 85, 0.94));
+  background: rgba(0, 0, 0, 0.75);
 }
 
 .bottom-nav {
@@ -2118,15 +2118,6 @@ textarea:focus-visible {
     min-height: 54px;
   }
 
-  .toast {
-    grid-template-columns: auto 1fr;
-  }
-
-  .toast__close {
-    grid-column: 2;
-    justify-self: end;
-  }
-
   .bottom-nav__inner {
     gap: 6px;
     padding: 10px;
@@ -2186,6 +2177,10 @@ textarea:focus-visible {
   padding: 20px;
   color: #64748b;
 }
+.loading-spinner svg,
+.is-spinning {
+  color: inherit;
+}
 
 @keyframes spin {
   to { transform: rotate(360deg); }