refactor(live-preview): migrate live-preview to signals

[spliffone]

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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 refactors the SiLivePreviewComponent to adopt Angular's signal-based inputs (input and model) instead of the traditional @input decorators, updating both the component class and its HTML template. The review feedback recommends migrating the remaining ticketBaseUrl input to the signal-based input() function to fully comply with the repository style guide. Additionally, the reviewer suggests caching signal reads (such as this.example() and this.baseUrl()) in local variables across multiple methods to avoid redundant evaluations and improve code readability.

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[dr-itz]

PR title is confusing. This is a signal migration (which is fine), not making in OnPush. We cannot mark live-preview as OnPush as it will break any examples that rely on Default. Everything is marked as Default and it should stay that way, at least for a few releases.

Having signals is the right thing to do as stuff will work in either case.