refactor(dashboards-ng): make editable a single source of truth

[chintankavathia]

Consolidates the dashboard grid's editable state by removing the duplicated editableInternal property in SiGridComponent and the editable$ BehaviorSubject in SiGridService. The editable model signal on SiGridComponent is now the sole owner of the editable state.

In SiWidgetHostComponent, the editable state is now passed as a regular input() binding instead of being consumed via an async observable from the grid service.

Why
Previously, the editable state was tracked in three places:

• The editable model signal (public API / input)
• A private editableInternal boolean in SiGridComponent
• A BehaviorSubject (editable$) on SiGridService

This made the code harder to reason about—edit(), save(), and cancel() had to keep all three in sync, and SiWidgetHostComponent subscribed to the service observable rather than receiving the value directly from its parent.

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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 dashboard grid's edit state management by replacing the BehaviorSubject in SiGridService and the internal editableInternal property with Angular signals and model inputs. While this modernizes the state handling, several regressions were identified where guards in the edit() and cancel() methods prevent necessary state transitions when triggered via input bindings. Additionally, feedback was provided to avoid any types in error handlers per the style guide and to ensure setupEditable is not called prematurely during the initial change detection cycle in SiWidgetHostComponent.