[WIP] Upgrade to Vue 3

[Copilot]

Upgrading Vue 2 to Vue 3 and replacing Vuex with Pinia in the admin frontend:

• Analyze current Vue 2 setup with Vuex 3.1.3
• Identify 45 store usage instances across components
• Verify current build works with legacy OpenSSL provider
• Update package.json with Vue 3 compatible versions
• Create new Pinia store to replace Vuex store
• Update main.js for Vue 3 compatibility (createApp, global properties)
• Convert global filters to global properties
• Update router configuration for Vue 3
• Fix router navigation guards to use Pinia properly
• Update core components: SideBar, HomeView, LoginView, InitialUserView, LanguageSelect
• Update remaining components: Table, ElepySingle, ElepyDefault, ActionButton
• Remove old Vuex store.js file
• Fix build issues with Vue 3 compatibility (UIKit asset paths)
• Test and fix any remaining compatibility issues
• Ensure build and linting work correctly

Progress:

• Successfully migrated all core store usage from Vuex to Pinia
• Updated all 14 instances of $store. usage to use Pinia actions/state
• Converted components from Options API with mappers to Composition API with Pinia
• Replaced Vuex mutations/actions with Pinia direct state/action calls
• Removed old store.js file completely
• Added UIKit asset handling for missing image files

Current issue: UIKit 3.3.6 CSS references images with absolute paths that need proper webpack configuration.

Fixes #180.

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