Frontend new way of dialogs

Each dialog managed by the dialog manager was only opened once and stayed in the DOM for the lifetime of the application. This causes:

• More memory usage: Dialogs accumulate in the DOM even when not visible
• More bugs because of missing state reset: Dialog state persists between opens, leading to stale data or unexpected behavior

We implemented a new way of handling dialogs using DialogMixin. With this approach:

• Dialogs are created when opened and destroyed when closed: No need to manually reset the state of the dialog when it is closed
• Closed events are automatically handled: The dialog mixin takes care of cleanup
• Subscribe mixin can now be used in dialogs: Since dialogs are properly destroyed, subscriptions are cleaned up automatically
• Use normal Lit lifecycle methods: Use connectedCallback to initialize when the dialog is opened instead of relying on the showDialog method

Check out ha-dialog-date-picker for a reference implementation. DialogMixin adds dialog params to this.params if available.