Fix element_info orphan row causing UNIQUE constraint errors on undo

Reviving #664, closed 2026-09-18 purely to clear a maintainer review
backlog (#630), not on merit; not superseded. Rewritten fresh against
current master rather than merged from the old branch -- that branch
predates the Qt6-only switch and much of dataBase/projectdatabase.cpp's
later rewrite, and the two had diverged too far for a textual merge to
be trustworthy.

projectDataBase::removeElement() only ran DELETE FROM element WHERE
uuid=:uuid. It never touched element_info, even though every element
also has a row there (element_uuid is its PRIMARY KEY, with a FOREIGN
KEY back to element.uuid that isn't enforced by this connection -- no
ON DELETE CASCADE in effect). So deleting an element left its
element_info row orphaned.

Re-adding an element with that same uuid later -- undo of that same
deletion, or a redo replaying it -- goes through addElement(), which
INSERTs into both tables. The element insert succeeds (that row really
was removed). The element_info insert hits the orphaned row's primary
key and fails, silently: the error is logged and swallowed, so the
element re-enters the scene with no element_info row at all, and
nothing later re-syncs it.

removeDiagram() already cascades this cleanup when a whole folio is
removed (a later, unrelated addition) -- confirmed on current master --
but that path never runs for a single element removed on its own,
which is the case this fixes.

Verified on the built binary, not just read: placed an element, deleted
it (Ctrl+A, Delete), undid the deletion (Ctrl+Z). Reverting just this
fix and repeating the identical sequence reproduces the exact reported
error:

  Debug: projectDataBase::addElement insert element info error :
  QSqlError("1555", "Unable to fetch row",
  "UNIQUE constraint failed: element_info.element_uuid")

With the fix, the same sequence produces nothing. Qt 6.10.2, ctest
13/13.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

sources/dataBase/projectdatabase.h

@@ -139,6 +139,7 @@ class projectDataBase : public QObject
 		QSqlQuery m_insert_elements_query,
 				  m_insert_element_info_query,
 				  m_remove_element_query,
+				  m_remove_element_info_query,
 				  m_update_element_query,
 				  m_insert_diagram_query,
 				  m_remove_diagram_query,