Admin Sidebar: support optional icons for extendable submenu

[MMilosz]

References

• Fixes Admin Sidebar: add (optional) icon support to extendable submenu items #6117

Description

This PR adds icon support to extendable submenu items in the admin sidebar.

Please note this PR does not add any new icons!

Example

Instructions for Reviewers

Changes in this PR:

• Added optional icon property to extendable submenu item configuration for every instance of MenuItemModel
• Updated submenu rendering logic to display icons when provided
• Updated MenuItemModels rendering logic to display icons when provided
  • Both AltmetricMenuItemModel and SearchMenuItemModel were not updated due to their specific use cases (they are not used in the extendable submenu anyway)
• Kept fallback behavior (existing menu items without icons still render correctly)
• Added tests (LinkMenuItem & OnclickMenuItem) to validate the icon rendering support

How to Test

Please note DSpace does not have built-in validation to verify if a FontAwesome icon string actually exists. This PR does not change that.

During tests, please ensure that:

• Admin Sidebar still behaves the same (no new icons, no changes in sections, etc.)
• New icons appear correctly when added via the icon property (check the configuration example below)
• No regression in menu rendering for items without icons
• Rendering fits DSpace standards (gaps between icon/label, etc.)

To fully test this PR, you can add new icons in submenu: go to any inheritor of AbstractExpandableMenuProvider and inside a model define a value for icon: '...'.

Example

// src/app/shared/menu/providers/new.menu.ts
// ...
return [
        {
          visible: isCommunityAdmin,
          model: {
            type: MenuItemType.ONCLICK,
            text: 'menu.section.new_community',
            icon: 'user-group', // <-------------- add an icon like this
            function: () => {
              this.modalService.open(ThemedCreateCommunityParentSelectorComponent);
            },
          },
        },
        // ...
]

LLM Disclosure

Instructed local Qwen3.8:27B to write tests & test compliance with CODE_CONVENTIONS.md & check browser compatibility before submitting the PR. The rest is my own work (meaning pretty much copy-pasted from the existing DSpace components 😉)

Checklist

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