desktop: tray menu panels and pinnable context menus

The tray's right-click menu is drawn by the app: one tree from tray.rs
(stable submenu keys), one frameless always-on-top menu-N window per
panel rendered by tray-menu.html with the design system, placed inside
the monitor's work area. A pure panel book keeps transient and pinned
panels apart; pinned panels (pin button, Ctrl+click, middle click) are
draggable, step aside from each other and are saved in state.json
(menuPins) and restored at start. tray_update pushes the tree to the
open panels, which update in place; closed panels are destroyed. The
native menu, rendered from the same tree, stays as the fallback and on
Linux. Only menu-* windows get the menu_* commands (menu.json).

In-app context menus get the same behaviour through a pure menu stack
(ui/menu-stack.ts): pin a menu, tear off a submenu, pinned panels
survive the next menu, are draggable and re-read their entries from
live sources so data changes update them in place.

Also drops a stray diff3 marker line from CHANGELOG.md.

apps/desktop/src-tauri/tauri.conf.json

@@ -50,7 +50,7 @@
         "form-action": "'none'"
       },
       "freezePrototype": true,
-      "capabilities": ["main-window", "card-window"]
+      "capabilities": ["main-window", "card-window", "menu-window"]
     }
   },
   "bundle": {