Fix welcome tab not restored in empty panes

When restoring session, panes that only had a welcome tab were left
empty because welcome tabs weren't saved. Now add a welcome tab to
any pane that ends up empty after restore.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/development_hub/workspace.py

@@ -601,6 +601,9 @@ class WorkspaceManager(QWidget):
         if widget_type == "pane":
             pane = self._create_pane()
             pane.restore_tabs(data.get("state", {}))
+            # If pane is empty after restore, add welcome tab
+            if pane.tab_count() == 0:
+                pane.add_welcome_tab()
             return pane
 
         elif widget_type == "splitter":