Add confirmation step when uninstalling a theme

Problem/Motivation

Currently, in Drupal 11, navigating to /admin/appearance allows users to uninstall an installed theme with a single click. Unlike the module administration page (/admin/modules/uninstall), there is no confirmation step.

This creates an inconsistency in the administrative user interface. If a user accidentally clicks "Uninstall," the theme is removed immediately. While the files remain on the server, any theme-specific settings or block placements associated with that theme might be lost or disrupted without warning.

Proposed resolution

Introduce a confirmation form (standard Drupal confirmation page) when a user clicks "Uninstall" for a theme, similar to the workflow used for modules.

Remaining tasks

User interface changes

Clicking "Uninstall" on a theme at /admin/appearance will redirect the user to a confirmation page. The page should warn the user that the theme will be uninstalled and ask for confirmation before proceeding.

API changes

None expected.

Data model changes

None