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/_focus-ring.scss

@@ -0,0 +1,15 @@
+// Focus ring mixin for keyboard accessibility
+$focus-ring-color: #1890ff;
+$focus-ring-width: 2px;
+$focus-ring-offset: 2px;
+
+@mixin focus-ring {
+  &:focus {
+    outline: none;
+    box-shadow: 0 0 0 $focus-ring-offset $focus-ring-color;
+  }
+  &:focus-visible {
+    outline: $focus-ring-width solid $focus-ring-color;
+    outline-offset: $focus-ring-offset;
+  }
+}