refactor: convert store to factory functions

[luka-nextcloud]

• Resolves: Multiple embed board widgets only show same board #7375
• Target version: main

Summary

Refactors the Vuex store setup to use factory functions for creating new store instances. This change allows Board reference widgets to maintain separate store instances, resolving the issue where loading multiple board widgets would always load the same board.

Store architecture refactor:

• Changed store/main.js to export a storeFactory function that returns a new Vuex store instance, instead of exporting a singleton store. All imports and usages of the store are updated to use storeFactory().

• Refactored store/stack.js and store/card.js to export factory functions (stackModuleFactory, cardModuleFactory) that return new module objects, replacing singleton module exports.

Widget and component store usage:

• Updated components and widgets (e.g., BoardReferenceWidget.vue, widget registration in init-reference.js) to use new store instances by calling storeFactory()

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Documentation (manuals or wiki) has been updated or is not required

[juliusknorr]

Could it be problematic here that we are not using the same instance as in the main app?