[WIP] Upgrade to Vue 3

[Copilot]

• [x] Update package.json dependencies (Vue 2→3, Vuex→Pinia, Vue Router 3→4, Vue I18n 8→9)
• [x] Update main.js to use Vue 3 createApp syntax
• [x] Convert store.js from Vuex to Pinia store
• [x] Update i18nSetup.js for Vue 3 compatibility
• [x] Update router.js for Vue Router 4 and Pinia
• [x] Update SideBar component to use Pinia instead of Vuex
• [ ] Fix remaining build errors (missing assets, component compatibility)
• [ ] Update remaining components using Vuex (mapState, mapGetters) to use Pinia
• [ ] Test build and functionality
• [ ] Fix any remaining Vue 3 compatibility issues

Fixes #180.

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