Update trello_cards: add drag and drop support for reordering in same column (#5934)

### Motivation

trello_cards: add drag and drop support for reordering in same column

### Implementation

- minor improvement of the example
- applied ruff code formatting

### Progress

- [x] The PR title is a short phrase starting with a verb like "Add
...", "Fix ...", "Update ...", "Remove ...", etc.
- [x] The implementation is complete. (Otherwise, open a [draft
PR](https://github.blog/news-insights/product-news/introducing-draft-pull-requests/).)
- [x] This PR does not address a security issue. (Security fixes must be
coordinated via the [security
advisory](https://github.com/zauberzeug/nicegui/security/advisories/new)
process before opening a PR.)
- [x] Pytests are not necessary.
- [x] Documentation is not necessary. 
- [x] No breaking changes to the public API

---------

Co-authored-by: Falko Schindler <falko@zauberzeug.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: 3 people

examples/trello_cards/draganddrop.py

@@ -11,6 +11,7 @@ class Item(Protocol):
 
 
 dragged: card | None = None
+target: card | None = None
 
 
 class column(ui.column):
@@ -32,13 +33,14 @@ def unhighlight(self) -> None:
         self.classes(remove='bg-blue-grey-3', add='bg-blue-grey-2')
 
     def move_card(self) -> None:
-        global dragged  # pylint: disable=global-statement # noqa: PLW0603
+        global dragged, target  # pylint: disable=global-statement # noqa: PLW0603
+        assert dragged is not None
         self.unhighlight()
-        dragged.parent_slot.parent.remove(dragged)
-        with self:
-            card(dragged.item)
-        self.on_drop(dragged.item, self.name)
+        dragged.move(self, self.default_slot.children.index(target) if target and target in self else -1)
+        if self.on_drop:
+            self.on_drop(dragged.item, self.name)
         dragged = None
+        target = None
 
 
 class card(ui.card):
@@ -49,7 +51,12 @@ def __init__(self, item: Item) -> None:
         with self.props('draggable').classes('w-full cursor-pointer bg-grey-1'):
             ui.label(item.title)
         self.on('dragstart', self.handle_dragstart)
+        self.on('dragover.prevent', self.handle_dragover)
 
     def handle_dragstart(self) -> None:
         global dragged  # pylint: disable=global-statement # noqa: PLW0603
         dragged = self
+
+    def handle_dragover(self) -> None:
+        global target  # pylint: disable=global-statement # noqa: PLW0603
+        target = self