feat(ui): Checkbox/Radio 原子件 + 3 处原生控件采用（P2-5）

- checkboxClass/radioClass（accent-cinnabar + 统一焦点环）+ 同名测试
- Checkbox/Radio 原子件：无 label 只渲染受控原生 input 便于就地替换
- 采用：ChainStarter 链类型单选、GeneratorRunner 入库勾选、CharacterCardItem 角色勾选

apps/web/components/chain/ChainStarter.tsx

@@ -5,6 +5,7 @@ import { Play } from "lucide-react";
 
 import { Button } from "@/components/ui/Button";
 import { Field } from "@/components/ui/Field";
+import { Radio } from "@/components/ui/Radio";
 import { SectionHeader } from "@/components/ui/SectionHeader";
 import { TextInput } from "@/components/ui/TextInput";
 import { CHAIN_KINDS, type ChainKind } from "@/lib/chain/chain";
@@ -58,8 +59,7 @@ export function ChainStarter({ onStart, disabled }: ChainStarterProps) {
               key={kind.key}
               className="flex items-start gap-2 text-sm text-ink"
             >
-              <input
-                type="radio"
+              <Radio
                 name="chain-kind"
                 value={kind.key}
                 checked={chainKey === kind.key}