refactor: UI/UX 全面重构 — 设计系统统一

设计系统：
- uni.scss 扩展：间距/圆角/字体/阴影/动画 token 体系
- 新建 styles/mixins.scss：20+ 公共 mixin

页面重构（12 个页面）：
- 首页/记账/统计/账单/个人中心/预算/分类管理
- 群组管理/通知中心/管理员/用户管理/编辑资料

所有硬编码替换为设计 token：
- 颜色：#FFF8F0→$bg, #FFFFFF→$surface, #FF8C69→$primary 等
- 间距：40rpx→$space-xl, 24rpx→$space-md 等
- 字体：28rpx→$font-lg, 24rpx→$font-md 等
- 圆角：20rpx→$radius-lg, 40rpx→$radius-2xl 等
- 阴影：统一使用 $shadow-md/$shadow-sm

重复样式提取为 mixin：
- page-base, sticky-header, nav-bar, card, modal-mask
- tabs, tab, state-box, shimmer, flex-center 等

client/src/pages/add/index.vue

@@ -216,51 +216,44 @@ function goBack() {
 </script>
 
 <style lang="scss" scoped>
+@import '@/styles/mixins.scss';
+
 .page {
-  min-height: 100vh;
-  background: #FFF8F0;
+  @include page-base;
   display: flex;
   flex-direction: column;
+  padding-bottom: 0;
 }
 
 .header-fixed {
-  position: sticky;
-  top: 0;
-  z-index: 100;
-  background: #FFF8F0;
+  @include sticky-header;
 }
 
 .status-bar {
-  background: #FFF8F0;
+  @include status-bar;
 }
 
 .header {
-  display: flex;
+  @include nav-bar;
   justify-content: space-between;
-  align-items: center;
-  padding: 16rpx 40rpx;
 }
 
 .nav-back {
-  width: 88rpx;
-  height: 88rpx;
-  display: flex;
-  align-items: center;
-  justify-content: center;
+  @include nav-back;
   border-radius: 50%;
 
-  &:active { background: #F0E0D6; }
+  &:active { background: $border; }
 }
 
-.title { font-size: 32rpx; font-weight: 500; color: #2D1B1B; }
+.title { font-size: $font-xl; font-weight: 500; color: $text; }
 
-.type-toggle-wrap { display: flex; justify-content: center; margin: 16rpx 0; }
+.type-toggle-wrap { display: flex; justify-content: center; margin: $space-sm 0; }
 
 .type-toggle {
   display: inline-flex;
-  background: #FFF0E6;
-  border-radius: 24rpx;
-  padding: 8rpx;
+  background: $surface-warm;
+  border-radius: $radius-lg;
+  padding: $space-xs;
   position: relative;
 }
 
@@ -270,48 +263,48 @@ function goBack() {
   display: flex;
   align-items: center;
   justify-content: center;
-  font-size: 28rpx;
-  color: #8B7E7E;
-  border-radius: 20rpx;
+  font-size: $font-lg;
+  color: $text-sec;
+  border-radius: $radius-lg;
   position: relative;
   z-index: 1;
-  transition: color 0.2s;
+  transition: color $transition-normal;
 
-  &.active { color: #FF8C69; font-weight: 600; }
+  &.active { color: $primary; font-weight: 600; }
 }
 
 .slider {
   position: absolute;
-  top: 8rpx;
-  left: 8rpx;
+  top: $space-xs;
+  left: $space-xs;
   width: 160rpx;
   height: 72rpx;
-  background: #FFFFFF;
-  border-radius: 20rpx;
-  box-shadow: 2rpx 2rpx 8rpx rgba(45, 27, 27, 0.08);
-  transition: transform 0.2s;
+  background: $surface;
+  border-radius: $radius-lg;
+  box-shadow: $shadow-sm;
+  transition: transform $transition-normal;
 }
 
 .income .slider { transform: translateX(160rpx); }
 
 .amount-display {
   text-align: center;
-  padding: 32rpx 40rpx 24rpx;
+  padding: $space-lg $space-xl $space-md;
 }
 
 .currency {
   font-family: 'Fredoka', sans-serif;
-  font-size: 48rpx;
+  font-size: $space-2xl;
   font-weight: 500;
-  color: #BFB3B3;
-  margin-right: 8rpx;
+  color: $text-muted;
+  margin-right: $space-xs;
 }
 
 .digits {
   font-family: 'Fredoka', sans-serif;
   font-size: 88rpx;
   font-weight: 700;
-  color: #2D1B1B;
+  color: $text;
   font-variant-numeric: tabular-nums;
 }
 
@@ -319,7 +312,7 @@ function goBack() {
   font-family: 'Fredoka', monospace;
   font-size: 88rpx;
   font-weight: 300;
-  color: #FF8C69;
+  color: $primary;
   animation: blink 1s step-end infinite;
 }
 
@@ -334,8 +327,8 @@ function goBack() {
 .category-grid {
   display: grid;
   grid-template-columns: repeat(4, 1fr);
-  gap: 16rpx;
-  padding: 8rpx 0;
+  gap: $space-sm;
+  padding: $space-xs 0;
 }
 
 .cat-item {
@@ -348,15 +341,15 @@ function goBack() {
   &:active { transform: scale(0.95); }
 }
 
-.cat-name { font-size: 22rpx; color: #8B7E7E; }
+.cat-name { font-size: $font-sm; color: $text-sec; }
 
 .cat-name.active {
-  color: #FF8C69;
+  color: $primary;
   font-weight: 600;
 }
 
 .extra-fields {
-  padding: 16rpx 40rpx;
+  padding: $space-sm $space-xl;
   display: flex;
   flex-direction: column;
   gap: 12rpx;
@@ -364,19 +357,19 @@ function goBack() {
 
 .extra-row {
   height: 88rpx;
-  background: #FFFFFF;
-  border-radius: 20rpx;
-  border: 2rpx solid #F0E0D6;
-  padding: 0 24rpx;
+  background: $surface;
+  border-radius: $radius-lg;
+  border: 2rpx solid $border;
+  padding: 0 $space-md;
   display: flex;
   align-items: center;
   gap: 12rpx;
 }
 
-.extra-text { font-size: 28rpx; color: #2D1B1B; flex: 1; }
-.extra-input { font-size: 28rpx; color: #2D1B1B; flex: 1; }
+.extra-text { font-size: $font-lg; color: $text; flex: 1; }
+.extra-input { font-size: $font-lg; color: $text; flex: 1; }
 
-.placeholder { color: #BFB3B3; }
+.placeholder { color: $text-muted; }
 
 @media (prefers-reduced-motion: reduce) {
   .cursor {