feat(desktop): wire New Window to ⌘⇧N + command palette

Repoint session.newWindow (⌘⇧N) from the compact new-session pop-out to
openNewWindow(), which opens a full peer instance via the new openWindow
bridge, and add a "New Window" entry to the ⌘K palette (shown with its
hotkey hint, gated on canOpenNewWindow()). Relabel the action "New window".

Drops the retired openNewSessionWindow bridge and the vestigial
isNewSessionWindow()/new=1 flag; renames the shared opener helper.

apps/desktop/src/lib/icons.ts

@@ -2,6 +2,7 @@ import {
   IconActivity as Activity,
   IconAlertCircle as AlertCircle,
   IconAlertTriangle as AlertTriangle,
+  IconAppWindow as AppWindow,
   IconArchive as Archive,
   IconArchiveOff as ArchiveOff,
   IconArrowUp as ArrowUp,
@@ -120,6 +121,7 @@ export {
   Activity,
   AlertCircle,
   AlertTriangle,
+  AppWindow,
   Archive,
   ArchiveOff,
   ArrowUp,