feat(ui): OS-native window controls + macOS titlebar overlay #1986

Parent

#1845 / #1843

Goal

Make window chrome feel native per OS:

• Windows / Linux: standard caption glyphs (Fluent-style min / max-restore / close), not Unicode improvisation.
• macOS: drop fake CSS traffic lights; use system controls via titleBarStyle: Overlay + hiddenTitle + trafficLightPosition, with the custom title bar content inset under the native lights (same idea as Flutter useNativeWindowChrome / no bitsdojo buttons on Mac).

Done when

• Win/Linux caption buttons use proper SVG icons and standard hover (close → red).
• macOS builds enable decorations + Overlay; frontend reserves left inset and does not paint fake lights.
• Drag region / menus still work; no double chrome.

How to work

• Branch: issue/<n>-os-window-controls from epic/tauri-migration.
• One PR → Closes #<n>, base epic/tauri-migration.