fix(upload): avoid skipping whole folder upload on conflict modal

[SergioChan]

Description

• fix conflict preflight for folder uploads so nested files are not all marked as conflicting when the top-level folder already exists
• keep file-level conflict detection unchanged for non-folder uploads
• this allows "Skip all conflicting files" to continue uploading non-conflicting files instead of aborting the transfer

Additional Information

• Repro from If I choose "Skip conflicting files", the transfer does not start #5798: re-uploading a partially uploaded folder surfaced the conflict modal, but selecting skip resulted in no upload starting because every nested file was incorrectly treated as a conflict.
• This patch is intentionally minimal and scoped to the preflight conflict check.

Checklist

• I am aware the project is currently in maintenance-only mode. See README
• I am aware that translations MUST be made through Transifex and that this PR is NOT a translation update
• I am making a PR against the master branch.
• I am sure File Browser can be successfully built. See builds and development.

Fixes #5798

Testing

• pnpm -C frontend install --config.engine-strict=false
• pnpm -C frontend run typecheck