Add a visible empty-file import error

[pranavdhawann]

Context

Empty Markdown or text files can enter the import pipeline but do not provide useful retrievable content, leaving the user with an unclear result.

Expected behavior

The importer rejects empty or whitespace-only files with a stable safe error code and the browser shows a useful retry-independent explanation.

Likely files

packages/importers/src/markdown.ts, packages/importers/src/text.ts, apps/api/src/services/import-service.ts, apps/web/src/features/imports/ImportWorkspace.tsx, colocated tests

Acceptance criteria

• Define one stable empty-source error code.
• Preserve other files in the same request.
• Show the filename and safe explanation in the UI.
• Add importer, API, and component regression tests.