Make yaak_api_client() read proxy settings from the database and apply
them to the reqwest client. Also replace bare reqwest::Client::new()
calls in license activate/deactivate with yaak_api_client() so they
get proxy support (and consistent UA/headers/timeout).
Fixes https://yaak.app/feedback/posts/cannot-activate-license
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Add new_xplatform_command() helper in yaak-common that creates a
tokio::process::Command with CREATE_NO_WINDOW flag set on Windows.
Also converts git commands to async for consistency.
