docs(desktop): define panel titlebar and sticky backdrop behavior

apps/desktop/DESIGN.md

@@ -202,6 +202,24 @@ Sizes: `default`, `xs`, `overlay` (titlebar glyph counts).
 - **No dividers between rows** unless the list genuinely needs them; prefer
   spacing. When you do need one, it's a single `--ui-stroke-tertiary` hairline.
 
+## Panel titlebars
+
+Top-edge panels extend into the native titlebar band. Their tab strips remain
+inside their own zones so tab drops, focus, and split boundaries use the same
+geometry. Lower panels keep local headers. Empty header space moves the window;
+tabs and actions remain no-drag, with native-control space reserved from the
+existing traffic-light and Window Controls Overlay measurements.
+
+The left cluster shows sidebar, settings, layout editor, and HUD controls. Flip
+and the right-sidebar toggle sit on the right; haptics remain in settings.
+Holding Cmd (Ctrl off macOS) reveals small slot numbers over the target strip's
+status dots after 400ms, without changing tab widths. Hints follow the same
+binding and hovered/focused-zone resolver as the number shortcuts.
+
+Sticky user messages mask scrolling content with the opaque chat surface,
+including the gap above them. Use `data-glass-opaque` so Glass cannot clear the
+mask; no gradient or backdrop blur.
+
 ## Feedback & empty/error/loading states
 
 - **Loading:** `Loader` (`src/components/ui/loader.tsx`) — animated math/ascii