fix: 构建 workspace 目录 4 层健康防护体系，彻底解决路径缺失问题

## 根本原因
workspace_create 只写 DB 不建目录，导致任何主题（小说/文章/海报等）
切换到目录已被删除/外置磁盘未挂载的 workspace 后，发送消息必然报错，
且旧提示语引导用户去不存在的"工作区设置"页面。

## 4 层防护

**L1 创建时（workspace_create）**
- 建 workspace 前调 ensure_workspace_root_ready，目录不存在则自动创建

**L2 启动/切换时（useWorkspace + useProjects）**
- refresh/setDefault/projects_refresh 时调 workspace_ensure_ready
- 静默自动修复已知 workspace 的缺失目录

**L3 发消息前（aster_agent_cmd）**
- aster_session_create + aster_agent_chat_stream 各自调 ensure_workspace_root_ready
- 能自动创建则创建并继续，无法创建则返回语义化错误

**L4 主动检查 + 兜底 UI（AgentChatPage + CrashRecoveryPanel）**
- 打开任意项目时主动调 workspace_ensure_ready，失败时显示 amber banner
- 发送失败也显示 amber banner，"重新选择目录"按钮处理两种场景
- CrashRecoveryPanel 新增 workspace-path-missing 场景识别与蓝色修复按钮

## 新增功能
- WorkspaceUpdate.root_path：支持通过 API 修改 workspace 根目录
- workspace_ensure_ready / workspace_ensure_default_ready：Tauri 健康检查命令
- workspace_health_service.rs：独立服务模块，含详细诊断信息
- AppCrashBoundary / CrashRecoveryPanel：通用 React 错误边界与恢复面板
- workspaceHealthTelemetry.ts：localStorage 修复历史记录（最多 50 条）

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/workspaceHealthTelemetry.ts

@@ -0,0 +1,82 @@
+export type WorkspaceRepairSource =
+  | "app_startup"
+  | "workspace_refresh"
+  | "workspace_set_default"
+  | "projects_refresh"
+  | "agent_chat_page";
+
+export interface WorkspaceRepairRecord {
+  timestamp: string;
+  workspace_id: string;
+  root_path: string;
+  source: WorkspaceRepairSource;
+}
+
+export interface RecordWorkspaceRepairInput {
+  workspaceId: string;
+  rootPath: string;
+  source: WorkspaceRepairSource;
+}
+
+const WORKSPACE_REPAIR_HISTORY_KEY = "proxycast.workspace_repair_history.v1";
+const MAX_WORKSPACE_REPAIR_HISTORY = 50;
+
+export function recordWorkspaceRepair(
+  input: RecordWorkspaceRepairInput,
+): void {
+  if (!input.workspaceId || !input.rootPath) return;
+
+  const nextRecord: WorkspaceRepairRecord = {
+    timestamp: new Date().toISOString(),
+    workspace_id: input.workspaceId,
+    root_path: input.rootPath,
+    source: input.source,
+  };
+
+  const history = readWorkspaceRepairHistory();
+  history.push(nextRecord);
+  const trimmed = history.slice(-MAX_WORKSPACE_REPAIR_HISTORY);
+  writeWorkspaceRepairHistory(trimmed);
+}
+
+export function getWorkspaceRepairHistory(limit = 50): WorkspaceRepairRecord[] {
+  if (limit <= 0) return [];
+  const history = readWorkspaceRepairHistory();
+  return history.slice(-limit).reverse();
+}
+
+export function clearWorkspaceRepairHistory(): void {
+  if (typeof localStorage === "undefined") return;
+  localStorage.removeItem(WORKSPACE_REPAIR_HISTORY_KEY);
+}
+
+function readWorkspaceRepairHistory(): WorkspaceRepairRecord[] {
+  if (typeof localStorage === "undefined") return [];
+
+  const raw = localStorage.getItem(WORKSPACE_REPAIR_HISTORY_KEY);
+  if (!raw) return [];
+
+  try {
+    const parsed = JSON.parse(raw);
+    if (!Array.isArray(parsed)) return [];
+    return parsed.filter(isWorkspaceRepairRecord);
+  } catch {
+    return [];
+  }
+}
+
+function writeWorkspaceRepairHistory(records: WorkspaceRepairRecord[]): void {
+  if (typeof localStorage === "undefined") return;
+  localStorage.setItem(WORKSPACE_REPAIR_HISTORY_KEY, JSON.stringify(records));
+}
+
+function isWorkspaceRepairRecord(value: unknown): value is WorkspaceRepairRecord {
+  if (typeof value !== "object" || value === null) return false;
+  const record = value as Record<string, unknown>;
+  return (
+    typeof record.timestamp === "string" &&
+    typeof record.workspace_id === "string" &&
+    typeof record.root_path === "string" &&
+    typeof record.source === "string"
+  );
+}