feat(web): 会话级角色 — 角色跟随激活会话 + 选角入口 + 面板挂角色（B 批次 1 前端）

按 DevDocs/design/22 §4.6/§5(7) 实现（后端由并行代理在做，按已定契约先行）。
- currentPersona 由「全局单值」改为「跟随激活会话」：新增 lib/sessionPersona.ts 解析（personaId 空 → 回落默认角色），App/TopBar/chat/lib/persona.ts 全部改为按激活会话取角色（头像/背景/名字/心情随会话切换）
- 交互：新建次级会话可选角色；会话换角色入口；管理面板 Sessions 页加「挂角色」下拉（走 PATCH /sessions/{id}）
- 组件拆分：Nav.tsx 228→143（抽出 NavItem.tsx 53）、新增 SessionRail.tsx 275（会话侧栏）
- 契约：types.ts / lib/api/sessions.ts 加 personaId?（建会话/改会话支持）
- mood 改为**按角色分桶**（同一角色的多个会话共享该角色心情）；升级后旧 mood 值不保留（刷新即空）——属行为变化，已在报告注明
- 测试：新增 personaSession.test.ts 40 checks（角色解析/回落/乐观更新与回滚）+ 既有 messageSync 54 checks 回归
- 验收：npm run build 绿；**未做浏览器端到端**（无 jsdom/渲染测试底座），UI 交互与失败路径提示未渲染验证
- 已知余量：useAppStore.ts 494/500 行（下次改动前应先拆 selectCurrentPersona/useCurrentPersona/useCurrentMood）

frontend/web/src/lib/api/sessions.ts

@@ -18,6 +18,9 @@ export interface MessageDto {
   deviceId?: string;
   /** Sender display name of a third-party message (design/16 §4.6). Blank for the owner. */
   senderName?: string;
+  /** The role that spoke this message (design/22 §7-5, backing column
+   *  `chat_messages.persona_id`); `''` means "inherit the current default role". */
+  personaId?: string;
   createdAt: number;
 }
 
@@ -35,6 +38,7 @@ function toChatMessage(m: MessageDto): ChatMessage {
     platform: m.platform,
     deviceId: m.deviceId,
     senderName: m.senderName,
+    personaId: m.personaId,
     createdAt: m.createdAt,
   };
 }
@@ -44,14 +48,54 @@ export function fetchSessions(): Promise<SessionMeta[]> {
   return request<SessionMeta[]>("/sessions");
 }
 
-/** POST /api/v1/sessions — creates a new secondary session. */
-export function createSubSession(title?: string): Promise<SessionMeta> {
-  return request<SessionMeta>("/sessions", {
+/** `POST /api/v1/sessions` reply: the new session's id, plus the bound role when
+ *  one was requested (docs/api-client.md §5) — the full summary is not echoed. `id`
+ *  is accepted too so a backend that returns a summary still reads correctly. */
+export interface CreateSessionResult {
+  sessionId?: string;
+  id?: string;
+  personaId?: string;
+}
+
+/** POST /api/v1/sessions — creates a new secondary session. `title` is the
+ *  optional display name; `personaId` binds the session to a role (design/22
+ *  §4.5). An omitted/empty `personaId` leaves the session inheriting the global
+ *  default role. The reply carries only the new id, so callers seed the new row
+ *  locally and let the next `GET /sessions` supply the metadata. */
+export function createSubSession(opts: {
+  title?: string;
+  personaId?: string;
+} = {}): Promise<CreateSessionResult> {
+  return request<CreateSessionResult>("/sessions", {
     method: "POST",
-    body: JSON.stringify(title ? { title } : {}),
+    body: JSON.stringify({
+      ...(opts.title ? { title: opts.title } : {}),
+      ...(opts.personaId ? { personaId: opts.personaId } : {}),
+    }),
   });
 }
 
+/** One PATCH /api/v1/sessions/{id} change set. At least one field must be set;
+ *  the gateway rejects an empty body with 400. `personaId: ""` explicitly
+ *  re-binds the session to "inherit the default role" (design/22 §4.5). */
+export interface SessionPatch {
+  title?: string;
+  pinned?: boolean;
+  personaId?: string;
+}
+
+/** PATCH /api/v1/sessions/{id} — updates a session's display metadata and/or its
+ *  bound role. The reply echoes only the fields that were changed. */
+export function patchSession(
+  sessionId: string,
+  patch: SessionPatch
+): Promise<{ id: string } & SessionPatch> {
+  return request<{ id: string } & SessionPatch>(
+    `/sessions/${encodeURIComponent(sessionId)}`,
+    { method: "PATCH", body: JSON.stringify(patch) }
+  );
+}
+
 /** GET /api/v1/sessions/{id}/messages?before=&limit=&platform= — hot-paginated
  *  history, optionally filtered to one source platform (design/16 §4.4). The wire
  *  returns newest-first, but callers render chronologically (oldest first), so we