refactor(mp): CSS 变量主题 + 登录页改造 — UI 优化 Phase 0-2

Phase 0: 建立 design token 体系
- tokens.scss 新增 --tk-pri/--tk-pri-l/--tk-pri-d/--tk-shadow-btn/--tk-shadow-tab
- .doctor-mode 覆盖为靛蓝色系，.elder-mode 非线性放大字号
- variables.scss 新增医生端色彩 + 阴影变量

Phase 1: 组件库 + 页面全局替换
- 75 个页面 SCSS $pri → var(--tk-pri) 全量替换
- 11 个新 UI 组件（PrimaryButton/TabFilter/FormInput/ProgressRing 等）
- 8 个现有组件 SCSS 更新
- 18 个医生端页面 useElderClass → useDoctorClass
- PageHeader 匹配原型 NavBar 规格

Phase 2: 登录页重写
- Logo: 方形+ → 圆形渐变 H
- 登录方式: 纯微信 → 账号密码 + 微信一键登录
- 新增 credentialLogin API + store action
- 字号/间距严格匹配原型 mp-01-login.html

apps/miniprogram/src/pages/pkg-profile/dialysis-prescriptions/detail/index.scss

@@ -5,7 +5,7 @@
   display: flex;
   justify-content: space-between;
   align-items: center;
-  margin-bottom: 8px;
+  margin-bottom: var(--tk-gap-xs);
 }
 
 .detail-title {
@@ -17,7 +17,7 @@
 
 .status-tag {
   display: inline-block;
-  padding: 4px 12px;
+  padding: var(--tk-gap-2xs) var(--tk-gap-sm);
   border-radius: $r-xs;
   font-size: var(--tk-font-body);
   font-weight: 500;
@@ -48,14 +48,14 @@
   font-weight: bold;
   color: $tx;
   display: block;
-  margin-bottom: 16px;
+  margin-bottom: var(--tk-gap-md);
 }
 
 .detail-row {
   display: flex;
   justify-content: space-between;
   align-items: flex-start;
-  padding: 12px 0;
+  padding: var(--tk-gap-sm) 0;
   border-bottom: 1px solid $bd-l;
 
   &:last-child {
@@ -74,7 +74,7 @@
   color: $tx;
   text-align: right;
   flex: 1;
-  margin-left: 24px;
+  margin-left: var(--tk-gap-lg);
   @include serif-number;
 }
 
@@ -88,7 +88,7 @@
   display: flex;
   justify-content: center;
   align-items: center;
-  padding: 120px 0;
+  padding: var(--tk-gap-2xl) 0;
 }
 
 .empty-text {

apps/miniprogram/src/pages/pkg-profile/dialysis-prescriptions/index.scss

@@ -5,23 +5,23 @@
 
 .page-title {
   @include section-title;
-  padding-left: 4px;
+  padding-left: var(--tk-gap-2xs);
 }
 
 .prescription-list {
   display: flex;
   flex-direction: column;
-  gap: 16px;
+  gap: var(--tk-gap-md);
 }
 
 .prescription-card {
   background: $card;
   border-radius: $r;
-  padding: 28px;
+  padding: var(--tk-card-padding-lg);
   box-shadow: $shadow-sm;
 
   &:active {
-    box-shadow: $shadow-md;
+    opacity: var(--tk-touch-feedback-opacity);
   }
 }
 
@@ -29,7 +29,7 @@
   display: flex;
   justify-content: space-between;
   align-items: center;
-  margin-bottom: 12px;
+  margin-bottom: var(--tk-gap-sm);
 }
 
 .prescription-model {
@@ -52,8 +52,8 @@
 
 .prescription-meta {
   display: flex;
-  gap: 24px;
-  margin-bottom: 8px;
+  gap: var(--tk-gap-lg);
+  margin-bottom: var(--tk-gap-xs);
 }
 
 .meta-item {