feat(projects): add safe workspace import and export

[luoye520ww]

Summary

• Add Import project and Export project actions to the existing workspace project picker.
• Copy project directories through the main process with an atomic staging directory and collision-safe destination names.
• Reject destinations inside the source project, skip symlinks and managed .kun/worktrees, and enforce file-count/size limits.
• Keep the transfer service in a lazy-loaded main-process chunk so the existing Electron main bundle remains stable.

This completes the project transfer portion of #878. Plan-mode model routing is covered separately by #895.

Safety

• Source and destination roots are canonicalized before the containment check.
• Transfers are staged under the selected parent and published with one rename.
• Existing destinations are never overwritten; a deterministic -copy-N name is selected.
• Symlinks and managed worktrees are skipped, and file count/byte limits fail closed.

Tests

npm.cmd exec vitest run src/main/services/project-transfer-service.test.ts src/main/ipc/app-ipc-schemas.test.ts src/renderer/src/components/chat/WorkspaceProjectPicker.test.ts
npm.cmd run typecheck
npm.cmd run lint -- --quiet
npm.cmd run build
git diff --check

Results: 57 targeted tests passed in CI; Typecheck, lint, build, and Linux/macOS/Windows package validation are green in CI. The old local worktree currently has no node_modules, so the commands were not rerun locally in this checkout.

Issue

Part of #878