fix: news feature security, accessibility, performance improvements

- DOMPurify strict allowlist with sandbox on iframes
- safeColor() CSS injection prevention for category_color
- URL validation (http/https) for images and links
- encodeURIComponent for slug in API calls
- Keyboard navigation on news cards (Enter/Space)
- aria-label, aria-pressed, type=button on all buttons
- 44px touch targets on remaining buttons
- memo() wrappers + stable callbacks (fewer re-renders)
- Canvas draw call batching (~75% reduction)
- TipTap extensions useMemo deps fix ([t] -> [])
- Featured image loading=eager fetchPriority=high (LCP)
- Unsafe Number(id) NaN guard

src/locales/zh.json

@@ -3953,6 +3953,7 @@
       "saveError": "保存文章失败，请重试。",
       "saved": "文章已保存",
       "deleted": "文章已删除",
+      "selectColor": "选择颜色 {{color}}",
       "toolbar": {
         "bold": "粗体",
         "italic": "斜体",