persist last-opened project and wire welcome open dialog

ProjectManager.open() now saves app.lastProject to user settings.
Boot calls openLast() instead of opening cwd — first launch shows
the welcome screen, subsequent launches restore the last project
directly into Claude. The welcome "Open project" button shows a
modal path-entry dialog that validates against git and activates
the Claude tab on success. Removed the dead "New Claude session"
button since secondary sessions are spawned via command palette.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/lib/main.dart

@@ -83,7 +83,7 @@ Future<void> main() async {
   await services.extensions.activateAll();
 
   if (!kIsWeb) {
-    final opened = await services.project.open(Directory.current.path);
+    final opened = await services.project.openLast();
     if (opened) {
       services.panels.activateTab(Slots.workspace, 'claude.primary');
     }