feat(mp): Phase 1 测试覆盖 + UX 无障碍 — 106 tests PASS + ARIA + focus ring

测试:
- secure-storage: 26 tests (AES 加解密/明文 fallback/迁移/Base64 边界)
- request.ts: 16 tests (扩展 ResponseCache/patientId 隔离/requestUnlimited)
- mock-api: 修复 getCachedPatientId 缺失导致 health 测试失败

UX 无障碍 (10 组件):
- SegmentTabs/DoctorTabBar: role=tablist/tab + aria-selected
- PrimaryButton/SecondaryButton: role=button + aria-disabled/aria-busy
- Loading/LoadingCard: role=status + aria-live=polite
- EmptyState: role=status + aria-live=polite
- ErrorState: role=alert + aria-live=assertive
- TrendChart tooltip: role=tooltip + aria-live=polite
- FormInput: aria-invalid + aria-label

焦点管理:
- 新增 _focus-ring.scss mixin (focus + focus-visible)
- 5 组件 SCSS 应用 focus-ring

apps/miniprogram/src/styles/variables.scss

@@ -68,3 +68,6 @@ $shadow-btn: 0 4px 16px rgba(196, 98, 58, 0.3);    // 主按钮阴影
 $shadow-tab: 0 2px 8px rgba(196, 98, 58, 0.25);     // 选中Tab阴影
 $shadow-btn-doc: 0 4px 16px rgba(58, 107, 140, 0.3); // 医生端按钮阴影
 $shadow-tab-doc: 0 2px 8px rgba(58, 107, 140, 0.25); // 医生端Tab阴影
+
+// ─── 焦点环（无障碍）───
+@import 'focus-ring';