Add configurable native glass window backgrounds

docs/WINDOW-MATERIALS.md

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+# Configurable window backgrounds
+
+Settings → Appearance offers Solid, Glass and Frosted glass. Preferences are
+saved locally. The tint-strength control ranges from 70 to 100%; the default
+is Frosted glass at 90%. Glass uses the direct tint; Frosted glass adds a 35%
+milk layer beneath it. This changes background treatment, never text opacity.
+
+Search input, result and preview panes and settings retain opaque reading
+surfaces. Shell secondary text becomes stronger over translucent backgrounds.
+High contrast immediately forces an opaque surface without changing the saved
+preference. Appearance writes are serialized; obsolete replies cannot overwrite
+the latest preference or accessibility state.
+
+## Platform support
+
+Windows 11 22H2 and newer use a transparent WebView and the established
+window-vibrancy crate's native acrylic implementation. Glass and Frosted glass
+share that native blur; their frontend milk tint differs. Desktop composition
+and OS high-contrast checks precede application, and native HRESULT failure
+returns an explicit solid fallback. Native window chrome remains enabled.
+
+Older Windows, macOS and Linux currently use a solid fallback. This macOS build
+does not enable private WebView transparency APIs. CSS backdrop-filter is not
+used to pretend the desktop is blurred. The settings report the effective
+material and explain the fallback.
+
+## Recorded checks, 2026-10-13
+
+- 57 frontend tests passed, including missing native capability, high contrast,
+  bounded tint, serialized pending preferences and stale failure handling.
+- Frontend type checking, production build and formatting passed.
+- 33 desktop Rust tests passed with one test thread; settings checks cover legacy
+  defaults, persisted preferences and invalid-value rejection preserving the
+  previous file and memory. An earlier parallel run hit a transient Windows
+  AccessDenied in an existing atomic-settings replacement test.
+- Desktop Rust formatting and strict all-target Clippy passed.
+
+Actual native material appearance and resize/compositor performance remain
+unverified. Enabled browser inventory was empty; native Windows capture/input
+access was unavailable. These checks establish implementation and persistence,
+not visual quality on release hardware. macOS/Linux native materials remain
+future work.