+ Project mới belongs to Project, not to every workspace screen

Moving it onto the title row put it on the wrong side of a boundary: that row
lives on WorkspaceTab, above the sub-tabs, so it is drawn over Cowork, Co4E,
Folder and GraphRAG too — a button offering to create a project in the corner of
a chat. The title label beside it already followed the sub-tab; the button did
not, because I added it to the row without adding it to that rule.

Both follow it now. check_project_screen walks the other four sub-tabs and fails
if the button shows on any of them.

23/23 checkers pass.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ui/workspace_tab.py

@@ -410,6 +410,10 @@ class WorkspaceTab(QWidget):
         # space taken from Co4E's canvas and Folder's tree on every laptop
         # screen. Shown where they apply; the text itself is unchanged.
         self._header.setVisible(on_project)
+        # The title row sits above the sub-tabs, so everything on it has to
+        # follow the same rule the title does — moving + Project mới up here put
+        # it in the corner of Cowork, Co4E, Folder and GraphRAG as well.
+        self._new_btn.setVisible(on_project)
         # ...and the explanation only while there is nothing to explain against:
         # the drawing heads a populated screen with the title alone.
         self._hint.setVisible(on_project and self.project_list.count() == 0)