sync: migrate batch4 attachment download + status button flow

[tyreseluo]

Summary

This PR migrates the batch4 attachment-download line from upstream into robrix2, with robrix2 as the base and conflict-minimized integration.

Included in this batch

• Added download icon resource and style constant (ICON_DOWNLOAD).
• Added ImageViewer download button support.
• Added unified attachment download module (shared/attachment_download.rs) for:
  • save dialog flow,
  • direct save for already-loaded image bytes,
  • timeline update callback support.
• Added message-level download section with status state:
  • download button for file/audio/video messages,
  • "Downloading..." state view,
  • per-timeline pending download tracking.
• Extended sliding sync request handling with DownloadMediaToFile.
• Added timeline update signal AttachmentDownloadFinished to clear pending state.
• Exposed MediaCache::timeline_update_sender() to wire download state callbacks.

Sync with latest main

• Updated local main from robrix2/main (f93beeaf).
• Merged latest main into this branch before pushing.

Validation

• cargo build passes.

Notes

• This is a low-conflict migration slice focused on attachment download and status-button behavior.