Fix macOS native tabs in unified titlebar

[fguisso]

Summary

• Place macOS native tabs beside the traffic lights in the compact unified titlebar
• Keep the tab layout stable when focus changes or AppKit rebuilds the titlebar
• Extend the liquid glass material into the titlebar so the window renders as one surface

Problem

NativeTab mode added a separate tab row below the macOS titlebar. With macOS liquid glass, the traffic lights, tabs, and terminal content were composed as separate surfaces, which created gaps and duplicated rounded corners.

Fix

The macOS window now repositions the native tab accessory with AppKit constraints and keeps those constraints updated when the titlebar changes. The integrated titlebar receives a matching glass view and follows the terminal background and opacity in active and inactive states. This behavior is limited to the macOS native tab and unified titlebar path.

Validation

• cargo +1.96.1 build -p rioterm --features wgpu
• cargo +1.96.1 test -p rio-window --lib
• cargo +1.96.1 clippy -p rio-window --lib -- -D warnings
• cargo +1.96.1 fmt --all -- --check