Data storage rewrite

[trobonox]

This is a tracking issue for the ongoing data storage rewrite, which should clean up some spaghetti of passing around things back and forth and centralize data which is accessed from lots of different places across the app.

This rewrite is a blocker for any other features or fixes (excluding security-critical fixes), so please keep in mind that until this is completed, there is a "feature freeze".

What needs to be done:

• Migrate settings to settings store
• Migrate theme to theme store
• Migrate layout properties (visibility of back arrow, help modal, changelog etc.) to layout store (WIP)
• Migrate board storage to store and add useBoard composable for easy management of one board
• Move management of background images to useBackgroundImage composable and separate BackgroundImage component (WIP)
• Adjust [id].vue to correctly use new stores (WIP)
• (optional but might be important) Separate Kanban page into smaller pieces
• Adjust home page to correctly use new stores (technically done but needs testing, reactivity seems to be broken e.g. when creating a new board!)
• Add created at dates to boards (and also old ones) to solve sorting issues on home page
• Adjust settings page to correctly use new stores
• Re-wire things to use only IDs instead of index (deletion, rename modals etc.) (WIP)
• Make sure imports/exports work correctly and keep boards store in sync