fix(dashboards-ng): preserve auto-positioned widget layout on cancel

[chintankavathia]

When widgets without explicit x/y are added, GridStack auto-positions them. On cancel, restoring configs with undefined x/y caused GridStack to re-auto-position, potentially producing a different layout.

Capture resolved positions from the DOM into persistedWidgetInstances when widgets are first added, so cancel() restores the exact layout.

Steps to reproduce:

1. Go to https://element.siemens.io/dashboards-demo/#/dashboard
2. Click edit
3. Reduce width of Pie Chart widget on second row
4. Shift "Full Speed" gauge chart widget left so that this widget and pie chart widget are next to each other
5. Click on cancel
6. Observe layout. "Full Speed" widget moves to third row

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Documentation.
Examples.
Dashboards Demo.
Playwright report.

Coverage Reports:

• charts-ng
• dashboards-ng
• element-ng
• element-translate-ng
• maps-ng
• native-charts-ng

[gemini-code-assist]

Code Review

This pull request introduces a mechanism to capture auto-resolved widget positions when they are first added to the grid, ensuring that a layout cancellation restores the exact positions. Feedback was provided regarding the performance of this implementation, specifically the potential for redundant DOM reads and array iterations during initial load. It was suggested to optimize the check by verifying if coordinates are actually missing and to use the editable() signal instead of the private editableInternal field to align with the project's style guide.