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/App.tsx

@@ -211,7 +211,7 @@ export function App({ onTheme }: { onTheme: () => void }) {
   const menu = useContextMenu();
   const appItem = (a: UnifiedApp, st: { text: string; tone: string }) => (
     <div role="listitem" key={a.key}>
-      <button {...menu(() => appMenu(a, d, nav, { showIgnored }))} className={`nav nav-app has-line${a.ignored ? ' is-ignored' : ''}`} aria-current={current({ screen: 'app', appId: a.key, tab: 'releases' })}
+      <button {...menu(() => appMenu(a, d, nav, { showIgnored }), `app:${a.key}`)} className={`nav nav-app has-line${a.ignored ? ' is-ignored' : ''}`} aria-current={current({ screen: 'app', appId: a.key, tab: 'releases' })}
         onClick={() => go({ screen: 'app', appId: a.key, tab: route.screen === 'app' && tabsOf(a).includes(route.tab) ? route.tab : 'releases' })}
         title={`${a.name} · ${st.text}`}>
         <span className="ds-side-ic"><AppIcon name={a.name} src={d.iconFor(a)} />{(st.tone === 'bad' || st.tone === 'warn') && <span className={`ds-sdot ${st.tone}`} role="img" aria-label={st.tone === 'bad' ? t('pf.blocker') : t('pf.warning')} />}</span>