fix scheduled import empty path handling

[charliecai]

Summary

• keep an empty scheduled-import path empty in the settings draft instead of displaying and persisting <project>/raw/sources
• initialize and hydrate newly opened projects with an empty path when no saved configuration exists
• clear project-managed paths left behind by older releases while retaining valid external selections
• make resolveImportPath() preserve empty values rather than interpreting them as raw/sources
• validate the whole project boundary, with distinct feedback for paths inside the project and paths that contain the project
• require a valid external directory before an enabled scheduled import configuration can be saved
• add English and Chinese copy plus regression coverage for empty, nested, parent, root, and external paths

Root cause

Both the settings draft and the project-opening flow replaced an empty scheduled-import path with <project>/raw/sources. The asynchronous hydration step only overwrote that value when a saved configuration existed, so a project without one retained the internal default. Saving unrelated settings or switching projects could then persist it. Separately, the runtime resolver also treated an empty path as raw/sources.

The previous managed-path check only covered selected project subdirectories, so an arbitrary path such as <project>/inbox was not rejected even though it is still inside the project.

User impact

Users with no scheduled-import directory configured now keep an empty store value and see an empty field prompting them to choose an external directory. Existing project-managed values are cleared during hydration and on the next settings save. Enabled configurations cannot be saved with a missing or self-referential path, and the UI explains whether the selected directory is inside the project or contains it.

Validation

• npm run test:mocks (118 files, 1739 tests)
• npm run build