Fix filenames across consecutive imports

[marpaia]

Summary

• Update the import name whenever a new file is accepted.
• Keep format inference and file-body loading unchanged.

Root cause and impact

The filename state was only initialized when it was empty. After the first upload, selecting or dropping another file retained the first filename and mislabeled the subsequent import.

Each consecutive import now uses the filename of the file that was actually selected.

Validation

• pnpm --dir apps/desktop lint
• pnpm --dir apps/desktop build

Fixes #4