feat: add accessibility config link to user menu footer section

[AlexAndBear]

Description

Related Issue

• Part of Add A11Y statement URL to user menu #858

How Has This Been Tested?

• test environment:
• test case 1:
• test case 2:
• ...

Types of changes

• Bugfix
• Enhancement (a change that doesn't break existing code or deployments)
• Breaking change (a modification that affects current functionality)
• Technical debt (addressing code that needs refactoring or improvements)
• Tests (adding or improving tests)
• Documentation (updates or additions to documentation)
• Maintenance (like dependency updates or tooling adjustments)

[Copilot]

Pull Request Overview

This PR adds an accessibility URL configuration to the user menu footer section by updating the theme URLs format, the user menu component, and associated unit tests.

• Changed the theme URL configuration in the mocks from an array to an object with keys including accessibility.
• Updated the UserMenu component to conditionally render the footer based on the availability of imprint, privacy, or accessibility URLs.
• Modified the theme validation schema to include an optional accessibility URL.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
packages/web-test-helpers/src/mocks/pinia.ts	Updated URL configuration structure to use an object with distinct keys, including accessibility.
packages/web-runtime/tests/unit/components/Topbar/UserMenu.spec.ts	Updated test cases and snapshot to check for the new accessibility URL in the footer.
packages/web-runtime/src/components/Topbar/UserMenu.vue	Modified footer rendering using a computed property to include accessibility URL.
packages/web-pkg/src/composables/piniaStores/theme.ts	Expanded the theme schema to optionally include an accessibility URL.

Comments suppressed due to low confidence (2)

packages/web-test-helpers/src/mocks/pinia.ts:116

• Switching from an array to an object for URL configuration improves extensibility; ensure that all dependent components and tests are updated accordingly.

        }

packages/web-pkg/src/composables/piniaStores/theme.ts:19

• [nitpick] If the accessibility URL is essential for functionality, consider enforcing its presence by making it required instead of optional.

      accessibility: z.string().optional()