fix: prevent worktree modal from disappearing after opening (#1585)

The worktree dialog state was local React state in SessionSidebar,
which gets destroyed when the component unmounts (mobile drawer close,
VSCode view switch). This caused the modal to briefly appear then
disappear.

Three changes:
- Move newWorktreeDialogOpen from local state to useUIStore so it
  survives component unmount
- Guard useProjectSessionSelection layout effect to skip when the
  worktree dialog is open (prevents auto-session-selection from
  closing the sidebar)
- Remove setSessionSwitcherOpen(false) from worktree button handler
  on mobile (prevents drawer close that unmounts the sidebar)

Fixes #1414

packages/ui/src/stores/useUIStore.ts

@@ -536,6 +536,7 @@ interface UIStore {
   isSessionCreateDialogOpen: boolean;
   isScheduledTasksDialogOpen: boolean;
   isSettingsDialogOpen: boolean;
+  isNewWorktreeDialogOpen: boolean;
   isModelSelectorOpen: boolean;
   sidebarSection: SidebarSection;
 
@@ -684,6 +685,7 @@ interface UIStore {
   setSessionCreateDialogOpen: (open: boolean) => void;
   setScheduledTasksDialogOpen: (open: boolean) => void;
   setSettingsDialogOpen: (open: boolean) => void;
+  setNewWorktreeDialogOpen: (open: boolean) => void;
   setModelSelectorOpen: (open: boolean) => void;
   applyTheme: () => void;
   setSidebarSection: (section: SidebarSection) => void;
@@ -824,6 +826,7 @@ export const useUIStore = create<UIStore>()(
         isSessionCreateDialogOpen: false,
         isScheduledTasksDialogOpen: false,
         isSettingsDialogOpen: false,
+        isNewWorktreeDialogOpen: false,
         isModelSelectorOpen: false,
         sidebarSection: 'sessions',
         settingsPage: 'home',
@@ -1495,6 +1498,10 @@ export const useUIStore = create<UIStore>()(
           });
         },
 
+        setNewWorktreeDialogOpen: (open) => {
+          set({ isNewWorktreeDialogOpen: open });
+        },
+
         setModelSelectorOpen: (open) => {
           set({ isModelSelectorOpen: open });
         },