fix(cli): add --overwrite to drive download and slides thumbnail

[WadydX]

Summary

Adds --overwrite flags to drive download and slides thumbnail --out, matching the existing behavior of photos download.

Closes #827

Problem

drive download and slides thumbnail --out silently overwrite existing local files because they route through createUserOutputFile, which hardcodes Overwrite: true.

drive download --tab is also affected because it delegates to runDocsTabExport, which uses the same helper.

Changes

• internal/cmd/drive_download.go

  • Add Overwrite bool to DriveDownloadCmd
  • Thread overwrite through downloadDriveFile(...) and runDocsTabExport(...)
  • Replace createUserOutputFile with openUserOutputFile(..., outputFileOptions{Overwrite: ...})

• internal/cmd/slides_thumbnail.go

  • Add Overwrite bool to SlidesThumbnailCmd
  • Thread overwrite through downloadSlidesThumbnail(...)
  • Replace createUserOutputFile with openUserOutputFile(..., outputFileOptions{Overwrite: ...})

• internal/cmd/docs_tab_export.go

  • Add Overwrite bool to tabExportParams
  • Replace createUserOutputFile with openUserOutputFile(..., outputFileOptions{Overwrite: ...})

Precedent

photos download already exposes --overwrite and passes Overwrite: c.Overwrite into openUserOutputFile.

Testing

Static analysis only. No local Go toolchain available to run make test.