Add macOS Share menu support for Quick Share

[hkay-dev]

AI-assisted: Codex helped write and review this code and PR.

Why

AirSync can send a file from its own Share control, but Finder and other macOS Share menus have no way to start that same transfer. This extension hands shared items to the existing Quick Share sender. It does not add another transfer implementation.

What changed

• Added a Share extension that accepts files, images, videos, text, and web links. It shows Send and Cancel controls, with an error message when it cannot prepare an item.
• The extension passes Finder file URLs to the containing app. It copies temporary provider data into the app group that the widget already uses, then opens the containing app with those URLs. Old staged copies are removed on a later share.
• Changed one line in the existing open-URLs handler to prefer the connected device. When there is no connected device, the existing device picker still handles the send. No Mac Quick Share protocol code or app settings changed.

share-service.mp4

Checks

• The unsigned AirSync Self Compiled build passes.
• A signed build of this branch appeared in Finder's Share menu. Its 43-byte test file arrived in Android Downloads through a side-by-side Android test build, with a matching SHA-256 hash. MediaStore reported is_pending=0.
• Two earlier consecutive sends also completed with the companion Android fixes.

Notes

• macOS controls Share menu placement, so the extension cannot force a spot above AirDrop.
• The existing sender does not support folders.
• Release signing needs the existing group.sameerasw.airsync-mac.widget app group enabled for the new Share extension identifier in the Apple Developer account.
• Companion Android PR: Fix Quick Share receiving and duplicate discovery airsync-android#157