Prioritize requested attachment downloads

[Xare123]

Summary

• Move a user-requested attachment ahead of older queued work in the same chat
• Preserve stable FIFO ordering for unrelated downloads
• Avoid duplicating an attachment already present in the queue
• Align the Dart and Rust priority request path

Why

When history contains many pending attachments, tapping one should prioritize that item instead of making the user wait for the backlog.

Validation

• flutter test test/services/network/attachment_download_queue_test.dart --reporter expanded
• 4 of 4 focused tests passed
• Combined validation with Retry transient sends and shared-profile downloads #250 and Improve photo loading and full-screen gallery swiping #253: 16 of 16 available tests passed
• Targeted analysis found no introduced errors
• git diff --check passed
• Hardened combined CI passed the complete Dart suite, real Android APK build, required native-library assertions, and artifact upload

Scope

This changes queue priority only. It does not alter attachment persistence or deletion behavior.