introduce first pinia store

[grnd-alt]

To eventually migrate to vue3 we should first introduce pinia.
luckily, we can use pinia and vuex at the same time so we can do rather easy partial migrations.

This pr shows how to do that by copying the boardStore we had in main.js before to it's own store.

The vuex stores depend on each other so some of them have to be migrated at the same time to not break things. The actionsStore however does not have any dependencies and no other store depends on it so it can be easily migrated in one step.

migration plan for other pinia stores

Store	Reads From (rootState/getters)	Commits To (other modules)
attachment.js	rootState.currentBoard	cardSetAttachmentCount, cardIncreaseAttachmentCount, cardDecreaseAttachmentCount
card.js	rootState.filter, rootState.stack.stacks, getters.stackById, getters.getSearchQuery	addStack, addBoard
stack.js	this.state.showArchived, this.state.currentBoard, rootState.card.cards	setCards, moveStackToTrash, updateCardProperty
trashbin.js	None	addStack, addCard
overview.js	None	setCurrentBoard, addCard
main.js	All modules	Provides currentBoard, filter, showArchived, board mutations
dashboard.js	None	None
comment.js	None	None
actions.js	None	None

dashboard, comment and actions (this pr) can be migrated it self
the others most likely have to be migrated together (we could write to vuex store from pinia as a workaround, but I don't think we should)

[mejo-]

Code changes look good now ☺️ Didn't test though.