Add configurable native glass window backgrounds

docs/ROADMAP.md

@@ -36,6 +36,10 @@ Status: implementation started; no production release yet. Windows VM tests use
   see [incremental semantic design](INCREMENTAL-SEMANTIC.md).
 
 ## 3. Desktop
+- [x] Persisted solid/glass/frosted appearance and bounded background tint;
+  opaque reading surfaces, serialized settings updates and high-contrast fallback.
+- [ ] Verify native glass visually on Windows 11 22H2+; macOS/Linux currently
+  use an explicit opaque fallback. Native compositor performance remains open.
 - [ ] Tauri shell, accessible keyboard search, progressive results and previews (shell built; UI interaction verification pending).
 - [ ] Folder selection, exclusion controls, resource budgets and indexing status.
 - [ ] Global shortcut, opening/revealing source files, PDF page navigation.
@@ -197,8 +201,10 @@ Audit and measured priorities: [EMBEDDINGGEMMA2-AUDIT.md](EMBEDDINGGEMMA2-AUDIT.
       media retry status across engine/desktop; genuine CPU promotion reuses
       identical document vectors with zero document re-encodes.
       See [early text setup](EARLY-TEXT-SETUP.md).
-    - [ ] Verify native first use during genuine delayed media download,
-      failure/retry and promotion; record actual first-input timing separately.
+    - [x] Native Linux CPU first use during genuine delayed media download,
+      failure/retry and promotion; actual first-input timing recorded separately.
+      Focused activation retest preserves selection and query process, records
+      fresh timing/current generation after idle. Windows/macOS checks stay open.
 - [ ] Buffered bounded media IPC and linear UTF-8 source-window offset processing.
 - [ ] Evaluate 256d MRL and content-specific vision budgets on independent data.
 - [ ] Validate GPU precision, synchronization-aware timings, parity and fallback