fix: save webview downloads natively + silence unused var warning

Combine and refine two community fixes:

- Save blob:/data:/Content-Disposition downloads via a native
  WebViewWindowBuilder::on_download handler instead of routing bytes
  through the Tauri IPC. The IPC origin (http://ipc.localhost) is blocked
  by strict-CSP sites (e.g. Gemini) and is unreachable from sandboxed
  iframes, so downloads silently failed while the page still reported
  success. The native path is independent of the page CSP and the IPC
  channel. (closes #1157)
- Drop the document.createElement("a") interceptor and let blob:/data:
  anchor clicks fall through to the native download path.
- Prefix the macOS-only `enable_find` binding with `_` to suppress the
  unused-variable warning on Windows/Linux builds.

Refinements on top of the original PRs:
- Toast on failed downloads too (not only success), so a failure is never
  silent again.
- Use `next_back()` instead of `last()` on the path-segment iterator.

Co-authored-by: Yushi47 <43625745+Yushi47@users.noreply.github.com>
Co-authored-by: reblox01 <74146687+reblox01@users.noreply.github.com>

https://claude.ai/code/session_018R2ceRrmzCUc13pSv9cXf5