Use a stock NSVisualEffectView for the macOS window backdrop

[Tryanks]

Behaviour

On macOS 27 the sidebar and window edges rendered with no blur: whatever sat behind the window showed through pin-sharp under the theme's 78% canvas.

GPUI's WindowBackgroundAppearance::Blurred inserts an NSVisualEffectView with the .selection material and strips its tint layers so only the Gaussian blur remains. On macOS 27 .selection carries no CABackdropLayer at all, so after the stripping the view paints nothing. Tracked with the layer dumps in #445, which waits on an upstream GPUI fix.

This PR stops relying on that path on macOS:

• The main window opens Transparent; run_shell slides a stock NSVisualEffectView (.sidebar, behind-window, follows window active state) under the GPUI view (crates/ui/src/macos_backdrop.rs). The material is left as AppKit builds it, so it follows Reduce Transparency and the macOS 27 Liquid Glass slider.
• The material's appearance is pinned to the theme mode in theme::change_mode, so a forced dark palette never sits on a light material.
• material::canvas returns transparent when the system material is installed, so the theme tint and the system tint do not stack. Overlay root and desktop chat root use it. Windows keeps the translucent canvas over Acrylic, which carries no tint of its own.
• raw-window-handle, objc2, objc2-app-kit become non-optional in tcode-ui (the backdrop is not a desktop-feature capability); the AppKit feature list gains NSVisualEffectView, NSView, NSWindow, NSAppearance, NSGraphics, NSResponder.
• docs/DESIGN.md Window material section and the T0 row describe the new composition.

Trade-off: the T0 tone is now the system's neutral gray rather than the theme's #F2F4F7; the strength is the user's, via the system slider.

Evidence

• cargo fmt --all --check, cargo clippy -p tcode-ui -p tcode --all-targets -- -D warnings: clean.
• cargo test -p tcode-ui -p tcode: 415 passed, 0 failed.
• Manual on macOS 27.0 (26A428): old build shows the window behind the sidebar sharp; this build shows a frosted sidebar with only soft colour bleeding through.
• Not run locally: cargo machete (not installed), iOS/Android/Web builds. raw-window-handle builds on wasm with std; relying on CI for those.