feat(overlay) use popover in overlay service

[wnvko]

Use popover global attribute to show the overlay. We are adding the attribute to overlay wrapper element and show the overlay by calling showPopover.

Closes #16824

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[Copilot]

Pull request overview

This PR modernizes the overlay service by adopting the native Popover API to manage element layering, replacing the traditional z-index approach. The implementation adds the popover="manual" attribute to overlay wrapper elements and uses showPopover()/hidePopover() methods to control visibility in the browser's top layer.

Changes:

• Added Popover API integration with showPopover() call before positioning and hidePopover() during cleanup
• Removed z-index styling in favor of browser-managed top layer positioning
• Added comprehensive style overrides to reset browser default popover styles while maintaining custom overlay positioning

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
projects/igniteui-angular/core/src/services/overlay/overlay.ts	Implements Popover API calls in show/hide logic with proper error handling and browser compatibility checks
projects/igniteui-angular/core/src/core/styles/components/overlay/_overlay-theme.scss	Removes z-index properties and adds popover attribute styles to override browser defaults

[dafo]

1. Run the demos locally
2. Navigate to http://localhost:4200/gridCellEditing
3. Double-click any row in the grid to open the editing dialog
4. While the editing dialog is opened - toggle the nav-drawer (no matter if you will open or close it)

Actual result:
The editing dialog position is broken

[damyanpetev]

1. Run the demos locally
2. Navigate to http://localhost:4200/gridCellEditing
3. Double-click any row in the grid to open the editing dialog
4. While the editing dialog is opened - toggle the nav-drawer (no matter if you will open or close it)

Actual result: The editing dialog position is broken

I would've thought the resize observer would handle that case, though it is possible to shift the position but that (without the native anchor positioning) I'd assume would be a problem in other cases too :/