[WIP] Upgrade to Vue 3

[Copilot]

• Update package.json dependencies (Vue 2→3, Vuex→Pinia, Vue Router 3→4, Vue I18n 8→9)
• Update main.js to use Vue 3 createApp syntax
• Convert store.js from Vuex to Pinia store
• Update i18nSetup.js for Vue 3 compatibility
• Update router.js for Vue Router 4 and Pinia
• 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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