fix: save right-click image/video downloads natively under strict CSP

The custom context-menu "save image/video" still routed blob:/data: media
through the Tauri IPC (download_file_by_binary), which strict-CSP sites such
as Gemini block at connect-src — the same root cause as #1157. Trigger a
native browser download via a transient anchor click instead, so the Rust
on_download handler saves it independently of the page CSP and the IPC.

This makes the native on_download path the single capture point for all
blob:/data: downloads, so the now-orphaned IPC machinery is removed:

- event.js: add triggerNativeDownload; use it for blob:/data: in
  downloadImage; drop the dead helpers (downloadFromDataUri,
  downloadFromBlobUrl, convertBlobUrlToBinary) and the
  collectUrlToBlobs/blobToUrlCaches createObjectURL override.
- invoke.rs / lib.rs: remove the unused download_file_by_binary command and
  its BinaryDownloadParams; drop the now-unused fs self-import.

https://claude.ai/code/session_018R2ceRrmzCUc13pSv9cXf5