fix(net): allow Accept-Language override

[xrip]

Summary

• keep en-US,en;q=0.9 as the default value
• allow each ObscuraHttpClient to set its Accept-Language value
• add a wire-level test for the override

This gives browser profile integrations a small upstream hook without changing current default behavior.

Closes #626.

Tests

• cargo check -p obscura-net
• cargo nextest run --release -p obscura-net --no-fail-fast (79 passed)