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/components/ui/PrimaryButton/index.scss

@@ -0,0 +1,54 @@
+@import '../../../styles/variables.scss';
+
+.primary-btn {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  gap: var(--tk-gap-xs);
+  width: 100%;
+  background: var(--tk-pri);
+  color: $white;
+  font-weight: 600;
+  border: none;
+  border-radius: 14px;
+  box-shadow: var(--tk-shadow-btn);
+  transition: opacity 0.15s, transform 0.15s;
+
+  &--default {
+    height: var(--tk-btn-primary-h);
+    font-size: var(--tk-font-body-lg);
+  }
+
+  &--large {
+    height: 54px;
+    font-size: var(--tk-font-h2);
+  }
+
+  &:active:not(&--disabled):not(&--loading) {
+    opacity: var(--tk-touch-feedback-opacity);
+    transform: scale(0.98);
+  }
+
+  &--disabled {
+    opacity: 0.5;
+    box-shadow: none;
+  }
+
+  &__spinner {
+    width: 18px;
+    height: 18px;
+    border: 2px solid rgba(255, 255, 255, 0.3);
+    border-top-color: $white;
+    border-radius: 50%;
+    animation: primary-btn-spin 0.6s linear infinite;
+  }
+
+  &__text {
+    color: $white;
+    font-weight: 600;
+  }
+}
+
+@keyframes primary-btn-spin {
+  to { transform: rotate(360deg); }
+}

apps/miniprogram/src/components/ui/PrimaryButton/index.tsx

@@ -0,0 +1,38 @@
+import React from 'react';
+import { View, Text } from '@tarojs/components';
+import './index.scss';
+
+interface PrimaryButtonProps {
+  children: React.ReactNode;
+  onClick?: () => void;
+  disabled?: boolean;
+  loading?: boolean;
+  size?: 'default' | 'large';
+  className?: string;
+}
+
+const PrimaryButton: React.FC<PrimaryButtonProps> = ({
+  children,
+  onClick,
+  disabled = false,
+  loading = false,
+  size = 'default',
+  className = '',
+}) => {
+  const cls = [
+    'primary-btn',
+    `primary-btn--${size}`,
+    disabled && 'primary-btn--disabled',
+    loading && 'primary-btn--loading',
+    className,
+  ].filter(Boolean).join(' ');
+
+  return (
+    <View className={cls} onClick={!disabled && !loading ? onClick : undefined}>
+      {loading && <View className='primary-btn__spinner' />}
+      <Text className='primary-btn__text'>{children}</Text>
+    </View>
+  );
+};
+
+export default React.memo(PrimaryButton);