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/screens/Ignored.tsx

@@ -30,7 +30,7 @@ export function Ignored() {
                 {apps.map(a => {
                   const s = d.studios.find(x => x.id === a.studio) ?? null;
                   return (
-                    <div className="li" key={a.key} {...cm(() => appMenu(a, d, nav, { showIgnored: true }))}>
+                    <div className="li" key={a.key} {...cm(() => appMenu(a, d, nav, { showIgnored: true }), `app:${a.key}`)}>
                       <AppIcon name={a.name} src={d.iconFor(a)} />
                       <div className="li-main">
                         <span className="li-title row" style={{ gap: 8 }}>{a.name}{!a.project && <StoreOnlyTag />}</span>