feat(web): 聊天背景渲染 + 思维链/工具卡富化

- persona.ts: 新增 personaChatBackground(p, orientation)，只取本角色对应方向的图
- ChatShell.tsx: 按横/竖屏渲染角色聊天背景（衬底+scrim），null/无图回退纯 Miuix 面
- chat.tsx: 思维链块加图标 chip+spinner+收起预览；工具卡加状态色 chip+ok/fail Tag
- 红线: 绝不 fallback 昔涟/其它角色；TemplateCard detail 折叠判定等行为不变

frontend/web/src/lib/persona.ts

@@ -8,3 +8,16 @@ const DEFAULT_AVATAR = "/images/User_Avatar/Default_Avatar.png";
 export function personaAvatar(p: PersonaRecord | null | undefined): string {
   return p?.avatar || DEFAULT_AVATAR;
 }
+
+/** Orientation for a persona's chat wallpaper. */
+export type Orientation = "landscape" | "portrait";
+
+/** Resolve the persona's own chat wallpaper for the given orientation. Returns
+ *  undefined when the persona has none (or is null) so callers keep the plain
+ *  Miuix surface. Never another role's image. */
+export function personaChatBackground(
+  p: PersonaRecord | null | undefined,
+  orientation: Orientation
+): string | undefined {
+  return p?.chatBackground?.[orientation];
+}